COMP519 Web Programming

Lecture 20: PHP (Part 2)

Handouts

Ullrich Hustadt

Department of Computer Science

School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

1 Comparisons

2 Arrays

Basics

Foreach-loops

Array Operators

3 Special types

NULL

Resources

4 Printing

5 Revision and Further Reading

COMP519 Web Programming Lecture 20 Slide L20 – 1

Comparisons

Comparison Operators

Type juggling also plays a role in the way PHP comparison operators work:

expr1 == expr2 Equal TRUE iﬀ expr1 is equal to expr2

after type juggling

expr1 != expr2 Not equal TRUE iﬀ expr1 is not equal to expr2

after type juggling

expr1 <> expr2 Not equal TRUE iﬀ expr1 is not equal to expr2

after type juggling

expr1 === expr2 Identical TRUE iﬀ expr1 is equal to expr2,

and they are of the same type

expr1 !== expr2 Not identical TRUE iﬀ expr1 is not equal to expr2,

or they are not of the same type

Note: For ==, !=, and <>, numerical strings are converted to numbers

and compared numerically

"123" == 123 ; TRUE "123" === 123 ; FALSE

"123" != 123 ; FALSE "123" !== 123 ; TRUE

"1.23e2" == 123 ; TRUE 1.23e2 === 123 ; FALSE

"1.23e2" == "12.3e1" ; TRUE "1.23e2" === "12.3e1" ; FALSE

"10hello5" == 10 ; TRUE "10hello5" === 10" ; FALSE

5 == TRUE ; TRUE 5 === TRUE ; FALSE

COMP519 Web Programming Lecture 20 Slide L20 – 2

Comparisons

Comparison Operators

Type juggling also plays a role in the way PHP comparison operators work:

expr1 < expr2 Less than TRUE iﬀ expr1 is strictly less than expr2

after type juggling

expr1 > expr2 Greater than TRUE iﬀ expr1 is strictly greater than expr2

after type juggling

expr1 <= expr2 Less than

or equal to

TRUE iﬀ expr1 is less than or equal to expr2

after type juggling

expr1 >= expr2 Greater than

or equal to

TRUE iﬀ expr1 is greater than or equal to expr2

after type juggling

Note: For >, >=, <, and <= numerical strings are converted to numbers

and compared numerically

'35.5' > 35 ; TRUE '35.5' >= 35 ; TRUE

'ABD' > 'ABC' ; TRUE 'ABD' >= 'ABC' ; TRUE

'1.23e2' > '12.3e1' ; FALSE '1.23e2' >= '12.3e1' ; TRUE

"F1" < "G0" ; TRUE "F1" <= "G0" ; TRUE

TRUE > FALSE ; TRUE TRUE >= FALSE ; TRUE

5 > TRUE ; FALSE 5 >= TRUE ; TRUE

COMP519 Web Programming Lecture 20 Slide L20 – 3

Comparisons

Comparison operators

•

To compare strings ‘as strings’ the strcmp function can be used

•

PHP 7 introduced the so-called ‘spaceship operator’ for three-way

comparisons (that converts numeric strings to numbers)

strcmp(expr1,expr2) String

comparison

Returns < 0 if expr1 is less than expr2,

> 0 if expr1 is greater than expr2,

0 if expr1 is equal to expr2

expr1 <=> expr2

(PHP 7 only)

Three-way

comparison

Returns -1 if expr1 < expr2,

+1 if expr1 > expr2,

0 if expr1 == expr2

strcmp('ABD','ABC') ; 1 strcmp("F1","G0") ; -65536

strcmp('aaa',"aaa") ; 0 strcmp('aaa',"AAA") ; 2105376

strcmp('1.23e2','12.3e1') ; -1

'ABD' <=> 'ABC' ; 1 "F1" <=> "G0" ; -1

'aaa' <=> "aaa" ; 0 'aaa' <=> "AAA" ; 1

'1.23e2' <=> '12.3e1' ; 0

'35.5' <=> 35 ; 1 '10hello5' <=> 10 ; 0

TRUE <=> FALSE ; 1 0.0 <=> FALSE ; 0

5 <=> TRUE ; 0 'FALSE' <=> TRUE ; 0

COMP519 Web Programming Lecture 20 Slide L20 – 4

Comparisons

Integers and Floating-point numbers: NAN and INF

NAN and INF can be compared with each other and other numbers using

equality and comparison operators:

NAN == NAN ; FALSE NAN === NAN ; FALSE NAN == 1 ; FALSE

INF == INF ; TRUE INF === INF ; TRUE INF == 1 ; FALSE

NAN < NAN ; TRUE INF < INF ; TRUE 1 < INF ; TRUE

NAN < INF ; TRUE INF < NAN ; TRUE INF < 1 ; FALSE

NAN < 1 ; TRUE 1 < NAN ; TRUE

In PHP 5.3 and earlier versions, INF == INF returns FALSE

INF === INF returns TRUE

In PHP 5.4 and later versions, INF == INF returns TRUE

INF === INF returns TRUE

COMP519 Web Programming Lecture 20 Slide L20 – 5

Comparisons

Integers and Floating-point numbers: NAN and INF

•

PHP provides three functions to test whether a value is or is not NAN,

INF or -INF:

•

bool is_nan(value)

returns TRUE iﬀ value is NAN

•

bool is_infinite(value)

returns TRUE iﬀ value is INF or -INF

•

bool is_finite(value)

returns TRUE iﬀ value is neither NAN nor INF/-INF

•

In conversion to a boolean value,

both NAN and INF are converted to TRUE

•

In conversion to a string,

NAN converts to 'NAN' and

INF converts to 'INF'

COMP519 Web Programming Lecture 20 Slide L20 – 6

Arrays Basics

Arrays

•

PHP only supports associative arrays (hashes), simply called arrays

•

PHP arrays are created using the array construct or,

since PHP 5.4, [ ... ]:

array ( key => value , ... )

[ key => value , ...]

where key is an integer or string and value can be of any type,

including arrays

$arr1 = [1 => " Peter " , 3 => 2009 , "a " => 101];

$arr2 = array (2008 46 36 9 = > array (" name " => " Jan Olsen " ,

" COMP51 8 " => 69 ,

" COMP51 9 " => 52 ));

•

The size of an array can be determined using the count function:

int count(array[, mode])

print count ( $arr1 ); // prints 3

print count ( $arr2 ); // prints 1

print count ( $arr2 ,1); // prints 4

COMP519 Web Programming Lecture 20 Slide L20 – 7

Arrays Basics

Arrays

•

It is possible to omit the keys when using the array construct:

$arr3 = array (" Peter " , "Paul " , " Mary " );

The values given in array will then be associated with the

natural numbers 0, 1, . . .

•

All the keys of an array can be retrieved using

array_keys($array1)

; returns a natural number-indexed array containing

the keys of $array1

•

All the values of an array can be retrieved using

array_values($array1)

; returns a natural number-indexed array containing

the values stored in $array1

COMP519 Web Programming Lecture 20 Slide L20 – 8

Arrays Basics

Arrays

•

An individual array element can be accessed via its key

•

Accessing an undeﬁned key produces a PHP notice

and returns NULL

$arr1 = array (1 = > "Peter " , 3 => 2009 , "a " => 101);

print " 'a ' = > " . $arr1 [ "a" ]. "\ n" ;

'a ' = > 101

print " 'b ' = > " . $arr1 [ "b" ]. "\ n" ;

PHP Notice : Undef in ed index : b in < file > on line < lineno >

'b ' = > // $arr1 [" b"] re turn s NULL

$arr1 [ 'b '] = 102;

print " 'b ' = > " . $arr1 [ "b" ]. "\ n" ;

'b ' = > 102

•

The function array_key_exists(key,array1) can be used to check

whether there is a value for key in array1

arr ay _key _e xists ("a ", $arr1 ) # ret urns TRUE

arr ay _key _e xists ("c ", $arr1 ) # ret urns FALSE

COMP519 Web Programming Lecture 20 Slide L20 – 9

Arrays Basics

Arrays

•

PHP allows the construct

$ array [] = value ;

PHP will determine the maximum value M among the integer indices in

$array and use the key K = M + 1; if there are no integer indices in

$array, then K = 0 will be used

; auto-increment for array keys

$arr4 [] = 51; // 0 = > 51

$arr4 [] = 42; // 1 = > 42

$arr4 [] = 33; // 2 = > 33

•

A key-value pair can be removed from an array using the

unset function:

$arr1 = array (1 = > "Peter " , 3 => 2009 , "a " => 101);

unset ( $arr1 [3]); // Removes the pair 3 => 2009

unset ( $arr1 ); // Removes the whole array

COMP519 Web Programming Lecture 20 Slide L20 – 10

Arrays Foreach-loops

Arrays: foreach-loop

•

PHP provides a foreach-loop construct to ‘loop’ through the elements

of an array

foreac h ( array as $ value )

stat eme nt

foreac h ( array as $ key = > $ value )

stat eme nt

•

array is an array expression

•

$key and $value are two variables, storing a diﬀerent key-value pair in

array at each iteration of the foreach-loop

•

We call $value the foreach-variable

•

foreach iterates through an array in the order in which elements were

deﬁned

COMP519 Web Programming Lecture 20 Slide L20 – 11

Arrays Foreach-loops

Arrays: foreach-loop

foreach iterates through an array in the order in which elements were

deﬁned

Example 1:

foreac h ( array (" Peter " , " Paul ", " Mary " ) as $ key => $ value )

print " The array maps $key to $value \ n";

The array maps 0 to Peter

The array maps 1 to Paul

The array maps 2 to Mary

Example 2:

$arr5 [2] = " Mary ";

$arr5 [0] = " Peter " ;

$arr5 [1] = " Paul ";

// 0 = > ' Peter ', 1 = > ' Paul ' , 2 = > ' Mary '

foreac h ( $arr5 as $ key = > $value )

print " The array maps $key to $value \ n";

The array maps 2 to Mary

The array maps 0 to Peter

The array maps 1 to Paul

COMP519 Web Programming Lecture 20 Slide L20 – 12

Arrays Foreach-loops

Arrays: foreach-loop

Does changing the value of the foreach-variable change the element of the

list that it currently stores?

Example 3:

$arr6 = array (" name " = > " Peter " , " year " = > 200 9);

foreac h ( $arr6 as $ key = > $value ) {

print " The array maps $key to $value \ n";

$value .= " - modified "; // Cha ngi ng $ v alue

}

print "\ n";

The array maps name to Peter

The array maps year to 2009

foreac h ( $arr6 as $ key = > $value )

print " The array now maps $key to $value \n ";

The array now maps name to Pete r

The array now maps year to 2009

COMP519 Web Programming Lecture 20 Slide L20 – 13

Arrays Foreach-loops

Arrays: foreach-loop

•

In order to modify array elements within a foreach-loop we need use a

reference

foreac h ( array as & $ value )

stat eme nt

unset ($ value );

foreac h ( array as $ key = > &$ value )

stat eme nt

unset ($ value );

•

In the code schemata above, $value is a variable whose value is stored at

the same location as an array element

•

PHP does not allow the key to be a reference

•

The unset statement is important to return $value to being a ‘normal’

variable

COMP519 Web Programming Lecture 20 Slide L20 – 14

Arrays Foreach-loops

Arrays: foreach-loop

•

In order to modify array elements within a foreach-loop we need use a

reference

•

In each iteration, $value contains a reference to an array element

; changing $value changes the array element

$arr6 = array (" name " = > " Peter " , " year " = > 200 9);

foreac h ( $arr6 as $ key = > & $ value ) { // Note : r efere nce !

print " The array maps $key to $value \ n";

$value .= " - modified ";

}

unset ( $value ); // Remove the re fer en ce from $value

print "\ n";

The array maps name to Peter

The array maps year to 2009

// See what the co nten t of $ arr6 is now

foreac h ( $arr6 as $ key = > $value )

print " The array now maps $key to $value \n ";

The array now maps name to Pete r - modi fie d

The array now maps year to 2009 - mod ified

COMP519 Web Programming Lecture 20 Slide L20 – 15

Arrays Foreach-loops

Array Assignments

•

In JavaScript arrays were objects and as a consequence array

assignments were done by reference

•

In PHP, this is not the case

$m em1 = memory _ g e t _ u s a ge ();

$ar ray1 = r ang e (1 , 1000);

$m em2 = memory _ g e t _ u s a ge ();

echo " (1) " ,spri ntf (" %6d " ,$ mem2 - $ mem1 )," more b yte s \n";

$ar ray2 = $ar ray1 ;

$m em3 = memory _ g e t _ u s a ge ();

echo " (2) " ,spri ntf (" %6d " ,$ mem3 - $ mem2 )," more b yte s \n";

$ar ray2 [1] += 1 0000 ;

echo "\ $a rray1 [1] = ", $a rray1 [1] ," | " ;

echo "\ $a rray2 [1] = ", $a rray2 [1] ,"\ n " ;

$m em4 = memory _ g e t _ u s a ge ();

echo " (3) " ,spri ntf (" %6d " ,$ mem4 - $ mem3 )," more b yte s \n";

(1) 36920 more bytes

(2) 0 more bytes

$ar ray1 [1] = 2 | $ arra y2 [1] = 10 002

(3) 36920 more bytes

The PHP implementation uses copy-on-write for array assignments

COMP519 Web Programming Lecture 20 Slide L20 – 16

Arrays Foreach-loops

Array Assignments

•

The PHP implementation uses copy-on-write for array assignments

•

If we want two array variables to point to the same array literal,

then we need to explicitly use a reference

$array 1 = range (1 , 100 0);

$mem2 = me mory _g et _usa ge ();

$array 2 = & $arra y1 ;

$mem3 = me mory _g et _usa ge ();

echo " (2) " ,s prin tf ( "%6 d" ,$mem3 - $mem2 ) ," more bytes \n ";

$array 2 [1] += 10000;

echo " \ $array1 [1] = " ,$ a rray 1 [1] , " | " ;

echo " \ $array2 [1] = " ,$ a rray 2 [1] , "\n ";

$mem4 = me mory _g et _usa ge ();

echo " (3) " ,s prin tf ( "%6 d" ,$mem4 - $mem3 ) ," more bytes \n ";

(2) 24 more bytes

$array 1 [1] = 10002 | $ a rray 2 [1] = 10002

(3) 0 more bytes

COMP519 Web Programming Lecture 20 Slide L20 – 17

Arrays Array Operators

Array Operators

PHP has no stack or queue data structures,

but has stack and queue operators for arrays:

•

array_push(&$array, value1, value2,...)

appends one or more elements at the end of the end of an array variable;

returns the number of elements in the resulting array

•

array_pop(&$array)

extracts the last element from an array and returns it

•

array_shift(&$array)

shift extracts the ﬁrst element of an array and returns it

•

array_unshift(&$array, value1, value2,...)

inserts one or more elements at the start of an array variable;

returns the number of elements in the resulting array

Note: &$array needs to be a variable

COMP519 Web Programming Lecture 20 Slide L20 – 18

Special types NULL

NULL

•

NULL is both a special type and a value

•

NULL is the only value of type NULL

and the name of this constant is case-insensitive

•

A variable has both type NULL and value NULL in the following three

situations:

1 The variable has not yet been assigned a value (not equal to NULL)

2 The variable has been assigned the value NULL

3 The variable has been unset using the unset operation

•

There are a variety of functions that can be used to test whether a

variable is NULL including:

•

bool isset($variable)

TRUE iﬀ $variable exists and does not have value NULL

•

bool is_null(expr)

TRUE iﬀ expr is identical to NULL

COMP519 Web Programming Lecture 20 Slide L20 – 19

Special types NULL

NULL

Warning: Using NULL with == may lead to counter-intuitive results

$d = array ();

echo v ar_ dum p ($d ), " \n" ;

array (0) {

}

echo ' is_null ( $d ): ' , ( is_ null ($d )) ? " TRUE \n " : " FALSE \ n";

is_nul l ($d ): FALSE

echo ' $d === null : ' , ( $d === null ) ? " TRUE \ n" : " FALSE \ n";

$d === null : FALSE

echo ' $d == null : ' , ( $d == null ) ? " TRUE \n" : " FALSE \ n";

$d == null : TRUE

Type juggling means that an empty array is (loosely) equal to NULL

but not identical (strictly equal) to NULL

COMP519 Web Programming Lecture 20 Slide L20 – 20

Special types Resources

Resources

A resource is a reference to an external resource

•

resource fopen(filename, mode)

Returns a ﬁle pointer resource for filename access using mode on

success, or FALSE on error

Mode Operation Create Truncate

’r’ read ﬁle

’r+’ read/write ﬁle

’w’ write ﬁle yes yes

’w+’ read/write ﬁle yes yes

’a’ append ﬁle yes

’a+’ read/append ﬁle yes

’x’ write ﬁle yes

’x+’ read/write ﬁle yes

See http://www.php.net/manual/en/resource.php for further details

COMP519 Web Programming Lecture 20 Slide L20 – 21

Special types Resources

Resources

•

bool fclose(resource)

•

Closes the resource

•

Returns TRUE on success

•

string fgets(resource [, length])

•

Returns a line read from resource and

returns FALSE if there is no more data to be read

•

With optional argument length, reading ends when length − 1 bytes

have been read, or a newline or on EOF (whichever comes ﬁrst)

•

string fread(resource,length)

•

Returns length characters read from resource

$handl e = fopen ( ' som efi le . txt ' , ' r ' );

while ( $line = fgets ( $ hand le )) {

// pro ce ssi ng the line of the file

}

fclose ( $ ha ndle );

COMP519 Web Programming Lecture 20 Slide L20 – 22

Special types Resources

Resources

•

int fwrite(resource, string [, length])

•

Writes a string to a resource

•

If length is given, writing stops after length bytes have been written or

the end of string is reached, whichever comes ﬁrst

•

int fprintf(resource, format, arg1, arg2, ...)

•

Writes a list of arguments to a resource in the given format

•

Identical to fprintf with output to resource

•

int vfprintf (resource, format, array)

•

Writes the elements of an array to a resource in the given format

•

Identical to vprintf with output to resource

$handl e = fopen ( ' som efi le . txt ' , ' w ' );

fwrite ( $handle ," Hello World !" . PHP_EOL ); // ` logic al n ewli ne '

fclose ( $ ha ndle );

In PHP \n always represents the character with ASCII code 10 not the

platform dependent newline ; use PHP_EOL instead

COMP519 Web Programming Lecture 20 Slide L20 – 23

Printing

In PHP, the default command for generating output is echo

•

void echo(arg1)

void echo arg1, arg2, ...

•

Outputs all arguments

•

No parentheses are allowed if there is more than one argument

•

More eﬃcient than print (and therefore preferred)

Additionally, PHP also provides the functions print, and printf:

•

int print(arg)

•

Outputs its argument

Only one argument is allowed!

•

Returns value 1

•

Parentheses can be omitted

COMP519 Web Programming Lecture 20 Slide L20 – 24

Printing

•

string sprintf(format, arg1, arg2, ....)

•

Returns a string produced according to the formatting string format

•

Parentheses are necessary

See http://www.php.net/manual/en/function.sprintf.php

for details

•

int printf(format, arg1, arg2, ...)

•

Produces output according to format

•

Parentheses are necessary

•

Returns the length of the outputted string

•

Important: A PHP array cannot take the place of a list of argu-

ments

printf ( "%2 d apples %2 d oran ges \ n" ,array (5 ,7));

produces a PHP warning and no output

COMP519 Web Programming Lecture 20 Slide L20 – 25

Printing

•

string vsprintf(format, array)

•

Returns a string produced according to the formatting string format

•

Identical to sprintf but accepts an array as argument

•

Parentheses are necessary

•

int vprintf(format, array)

•

Produces output according to format

•

Identical to printf but accepts an array as argument

•

Parentheses are necessary

vprint f (" %2 d apples %2 d orang es \ n" ,array (5 ,7));

5 apples 7 o rang es

COMP519 Web Programming Lecture 20 Slide L20 – 26

Revision and Further Reading

•

Read

•

Chapter 4: Expressions and Control Flow in PHP: Operators

•

Chapter 6: PHP Arrays

of R. Nixon: Learning PHP, MySQL & JavaScript:

with jQuery, CSS & HTML5. O’Reilly, 2018.

•

Read

•

Language Reference: Types: Arrays

http://uk.php.net/manual/en/language.types.array.php

•

Language Reference: Control Structures: foreach

http://uk.php.net/manual/en/control-structures.foreach.php

of P. Cowburn (ed.): PHP Manual. The PHP Group, 25 Oct 2019.

http://uk.php.net/manual/en [accessed 26 Oct 2019]

COMP519 Web Programming Lecture 20 Slide L20 – 27