COMP284 Scripting Languages
Lecture 5: PHP (Part 3)
Handouts
Ullrich Hustadt
Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science
University of Liverpool
Contents
1 Control Structures
Statement Groups
Conditional Statements
Switch Statements
While- and Do While-loops
For-loops
Try, throw, catch, finally statements
2 Functions
Defining a function
Calling a function
Scope of Variables
Functions and HTML
Variable-length argument lists
3 PHP libraries
Include/Require
COMP284 Scripting Languages Lecture 5 Slide L5 – 1
Control Structures Statement Groups
Control Structures: Statement Groups
A statement group is a sequence of zero or more statements delimited
by a pair of curly brackets
{
statements
}
It allows to use multiple statements where PHP expects only one
statement
{
$x = 1;
$y = $x ++;
}
COMP284 Scripting Languages Lecture 5 Slide L5 – 3
Control Structures Conditional Statements
Conditional Statements
The general format of conditional statements is very similar but not
identical to that in Java and JavaScript:
if ( condition )
statement
elseif ( condition )
statement
else
statement
•
the elseif-clause is optional and there can be more than one
Note: elseif instead of elif or else if
•
the else-clause is optional but there can be at most one
COMP284 Scripting Languages Lecture 5 Slide L5 – 4
Control Structures Conditional Statements
Conditional Statements/Expressions
•
PHP allows to replace curly brackets with a colon : combined with an
endif at the end of the statement:
if ( condition ):
statements
elseif ( condition ):
statements
else :
statements
endif
This also works for the switch statement in PHP
However, this syntax becomes difficult to parse
when nested conditional statements are used and is best avoided
•
PHP also supports conditional expressions
condition ? if_tru e _ e x pr : if_fa l s e _e x p r
COMP284 Scripting Languages Lecture 5 Slide L5 – 5
Control Structures Switch Statements
Switch Statement
A switch statement in PHP takes the following form
switch ( expr ) {
case expr1 :
statements
break ;
case expr2 :
statements
break ;
defaul t :
statements
break ;
}
•
there can be arbitrarily many case-clauses
•
the default-clause is optional but there can be
at most one, it does not need to be last
•
expr is evaluated only once and then compared
to expr1, expr2, etc using (loose) equality ==
•
once two expressions are found to be equal the
corresponing clause is executed
•
if none of expr1, expr2, etc are equal to expr,
then the default-clause will be executed
•
break ‘breaks out’ of the switch statement
•
if a clause does not contain a break command,
then execution moves to the next clause
COMP284 Scripting Languages Lecture 5 Slide L5 – 6
Control Structures Switch Statements
Switch Statement: Example
Not every case-clause needs to have associated statements
switch ( $ month ) {
case 1: case 3: case 5: case 7:
case 8: case 10: case 12:
$days = 31;
break ;
case 4: case 6: case 9: case 11:
$days = 30;
break ;
case 2:
$days = 28;
break ;
defaul t :
$days = 0;
break ;
}
COMP284 Scripting Languages Lecture 5 Slide L5 – 7
Control Structures While- and Do While-loops
While- and Do While-loops
PHP offers while-loops and do while-loops
while ( condition )
statement
do
statement
while ( condition );
Example:
// Compute the factorial of $number
$factorial = 1;
do
$factorial *= $number - -;
while ( $ number > 0);
COMP284 Scripting Languages Lecture 5 Slide L5 – 8
Control Structures For-loops
For-loops
•
for-loops in PHP take the form
for ( initia l i s at i on ; test ; increment )
statement
•
In PHP initialisation and increment can consist of more than one
statement, separated by commas instead of semicolons
Example:
for ( $i = 3, $j = 3; $j >= 0; $i ++ , $j - -)
echo " $i - $j - " , $i * $j , "\n" ;
3 - 3 - 9
4 - 2 - 8
5 - 1 - 5
6 - 0 - 0
COMP284 Scripting Languages Lecture 5 Slide L5 – 9
Control Structures For-loops
Break and Continue
•
The break command can also be used in while-, do while-, and for-loops
and discontinues the execution of the loop
while ( $value = array_shift ( $data ) {
$written = fwrite ( $resource , $value );
if (! $ written ) break ;
}
•
The continue command stops the execution of the current iteration of a
loop and moves the execution to the next iteration
for ( $x = -2; $x <= 2; $x ++) {
if ( $x == 0) continue ;
printf ( " 10 / %2 d = %3 d \n" ,$x ,(10/ $x ));
}
10 / -2 = -5
10 / -1 = -10
10 / 1 = 10
10 / 2 = 5
COMP284 Scripting Languages Lecture 5 Slide L5 – 10
Control Structures Try, throw, catch, finally statements
Exceptions and error handling
•
PHP distinguishes between Exceptions and Errors
•
In PHP 7 both are subclasses of Throwable but not in PHP 5
•
Exceptions can be thrown via a throw statement
throw new E x c e ption ( message )
where message is a string
•
User-level errors/warnings/notices can be generated using
trigger_e r r o r ( message [, type ])
where message is a string and type is an integer indicating what type
of error is generated
•
A try ... catch ... statement allows for exception (throwable)
handling
•
Since PHP 5.5, a finally clause can be added
try { statements }
catch ( Exception $e ) { s t a t e m e n t s }
finall y { statements }
COMP284 Scripting Languages Lecture 5 Slide L5 – 11
Control Structures Try, throw, catch, finally statements
Exceptions and error handling
$x = "A" ;
try {
if (( is_in t ( $x ) || is_float ($x )) && is_finite ( $x ))
$y = round ($x ,2);
else
throw new E x c e ption ( "' $x ' is not a number " );
} catch ( Exception $e ) {
echo " Caught : $e \n" ;
$y = 0;
} finally {
echo "y = $y \ n";
}
Caught : Exception : ' A ' is not a number in try . php :7
Stack trace :
#0 { main }
y = 0
COMP284 Scripting Languages Lecture 5 Slide L5 – 12
Control Structures Try, throw, catch, finally statements
Exceptions and error handling
•
PHP distinguishes between exceptions and errors
•
In PHP 5, errors must be dealt with by an error handling function
(‘Division by zero’ produces an error not an exception)
•
To take advantage of try ... catch ... statements to handle errors,
one can turn errors into exceptions
function ex c ep ti on_error _ ha nd ler ( $errno , $errstr ,
$errfile , $errline ) {
throw new E rr o rE x ce p t io n ( $errstr , $ errno ,
0, $errfile , $errline ); }
set_e r r or _ ha nd le r (" excep t io n_ er ror_hand l er " );
$x = 0;
try {
$y = 1/ $x ;
} catch ( Exception $e ) {
echo " Caught : $e \n" ;
}
Caught : E r ro r E xc e pt i on : Division by zero in try . php :10
COMP284 Scripting Languages Lecture 5 Slide L5 – 13
Functions Defining a function
Functions
Functions are defined as follows in PHP:
function identifier ( $ param1 , & $ param2 , ...) {
statements
}
•
Functions can be placed anywhere in a PHP script but preferably they
should all be placed at start of the script (or at the end of the script)
•
Function names are case-insensitive
•
The function name must be followed by parentheses
•
A function has zero, one, or more parameters that are variables
•
Parameters can be given a default value using
$param = const_expr
•
When using default values, any defaults must be on the right side of
any parameters without defaults
COMP284 Scripting Languages Lecture 5 Slide L5 – 14
Functions Defining a function
Functions
Functions are defined as follows in PHP:
function identifier ( $ param1 , & $ param2 , ...) {
statements
}
•
The return statement
return value
can be used to terminate the execution of a function and to make
value the return value of the function
•
The return value does not have to be scalar value
•
A function can contain more than one return statement
•
Different return statements can return values of different types
COMP284 Scripting Languages Lecture 5 Slide L5 – 15
Functions Calling a function
Calling a function
A function is called by using the function name followed by a list of
arguments in parentheses
function identifier ( $ param1 , &$ param2 , ...) {
...
}
... identifier ( arg1 , arg2 ,...) ...
•
The list of arguments can be shorter as well as longer as
the list of parameters
•
If it is shorter, then default values must have been specified for the
parameters without corresponding arguments
•
If no default values are available for ‘missing’ arguments,
then we get a PHP fatal error
Example:
function sum ( $num1 , $num2 ) { return $num1 + $num2 ; }
echo " sum : " ,sum (5 ,4) , "\n";
$sum = sum (3 ,2);
COMP284 Scripting Languages Lecture 5 Slide L5 – 16
Functions Scope of Variables
Variables and Scope
PHP distinguishes the following categories of variables with respect to
scope:
•
Local variables are only valid in the part of the code
in which they are introduced
•
(PHP) Global variables are valid outside of functions
•
Superglobal variables are built-in variables that are always available in
all scopes
•
Static variables are local variables within a function that retain their
value between separate calls of the function
By default,
•
variables inside a function definition are local but not static
•
variables outside any function definition are global
COMP284 Scripting Languages Lecture 5 Slide L5 – 17
Functions Scope of Variables
Functions and Scope
$x = " Hello ";
function f1 () {
echo " 1: $x \n" ;
}
function f2 () {
echo " 2: $x \n" ;
$x = " Bye " ;
echo " 3: " ,$x ," \ n";
}
f1 ();
f2 ();
echo " 4: $x \n" ;
•
A variable defined outside any
function is (PHP) global
•
A (PHP) global variable can be
accessed from any part of the
script that is not inside a function
•
A variable within a PHP function is
by default local and can only be
accessed within that function
•
There is no hoisting of variable
‘declarations’
1: PHP Notice : Undefined variable : x
2: PHP Notice : Undefined variable : x
3: Bye
4: Hello
COMP284 Scripting Languages Lecture 5 Slide L5 – 18
Functions Scope of Variables
Functions and Global Variables
$x = " Hello ";
function f1 () {
global $x ;
echo " 1: $x \n" ;
}
function f2 () {
$x = " Bye " ;
echo " 2: $x \n" ;
global $x ;
echo " 3: $x \n" ;
}
f1 ();
f2 ();
echo " 4: $x \n" ;
1: Hello
2: Bye
3: Hello
4: Hello
•
A ‘local’ variable can be declared
to be (PHP) global using the
keyword global
•
All (PHP) global variables with
the same name refer to the same
storage location/data structure
•
An unset operation removes a
specific variable, but leaves other
(global) variables with the same
name unchanged
COMP284 Scripting Languages Lecture 5 Slide L5 – 19
Functions Scope of Variables
PHP functions and Static variables
•
A variable is declared to be static using the keyword static and should
be combined with the assignment of an initial value (initialisation)
function counter () {
static $count = 0;
return $count ++;
}
; static variables are initialised only once
1 function counter () { s tatic $ count = 0; return $count ++; }
2 $co unt = 5;
3 echo " 1: global \ $ count = $ count \n" ;
4 echo " 2: static \ $ count = " , c ounter () , " \n";
5 echo " 3: static \ $ count = " , c ounter () , " \n";
6 echo " 4: global \ $ count = $ count \n" ;
1: global $ count = 5
2: static $ count = 0
3: static $ count = 1
4: global $ count = 5
COMP284 Scripting Languages Lecture 5 Slide L5 – 20
Functions Scope of Variables
PHP functions: Example
function bubble_sort ( $ array ) {
// $ array , $size , $i , $j are all local
if (! is_array ( $a rray ))
trigger_e r r o r (" Arg u m e nt not an array \n" , E_U S E R _ ER R O R );
$size = count ( $ array );
for ( $i =0; $i < $size ; $i ++) {
for ( $j =0; $j < $size -1 - $i ; $j ++) {
if ( $ar ray [ $j +1] < $arra y [ $j ]) {
swap ( $array , $j , $j +1); } } }
return $array ;
}
function swap (& $array , $i , $j ) {
// swap expects a reference ( to an array )
$tmp = $array [ $i ];
$array [ $i ] = $ array [$j ];
$array [ $j ] = $ tmp ;
}
COMP284 Scripting Languages Lecture 5 Slide L5 – 21
Functions Scope of Variables
PHP functions: Example
function bubble_sort ( $ array ) {
... swap ( $array , $j , $j +1); ...
return $array ;
}
function swap (& $array , $i , $j ) {
$tmp = $array [ $i ];
$array [ $i ] = $ array [$j ];
$array [ $j ] = $ tmp ; }
$array = array (2 ,4 ,3 ,9 ,6 ,8 ,5 ,1);
echo " Before sorting " , join (", " , $arr ay ) , " \ n";
$sorted = bubble_sort ( $ array );
echo " After s orting " , join (" , " ,$array ) , " \ n";
echo " Sorted array ", join ( ", ", $sor t ed ) , " \ n";
Before sorting 2, 4, 3, 9, 6 , 8 , 5 , 1
After s orting 2, 4 , 3 , 9 , 6 , 8 , 5 , 1
Sorted array 1, 2, 3, 4, 5, 6, 8, 9
COMP284 Scripting Languages Lecture 5 Slide L5 – 22
Functions Functions and HTML
Functions and HTML
•
It is possible to include HTML markup in the body of a
function definition
•
The HTML markup can in turn contain PHP scripts
•
A call of the function will execute the PHP scripts, insert the output
into the HTML markup, then output the resulting HTML markup
<?php
fu nct io n print _f orm ( $fn , $ ln ) {
?>
< form ac tion = " proces s_ fo rm .php " method = POST " >
<label > Fi rst Name : <inpu t type = " tex t " name =" f" v alu e =" <? php e cho $fn ? >" ></ label >
<br > < label > Last Name <b >* </b >:< input type =" te xt " nam e =" l " value = " <? php echo $ln ? > ">
</label ><br > < input type =" submit " name =" subm it " value =" Subm it " > <inpu t type = reset >
</form >
<?php
}
pr int_for m ( " Ul lrich ", " Hu sta dt ");
?>
< form ac tion = " proces s_ fo rm .php " method = POST " >
<label > Fi rst Name : <inpu t type = " tex t " name =" f" v alu e =" Ullri ch " ></ label >
<br > < label > Last Name <b >* </b >:< input type =" te xt " nam e =" l " value = " Hu stadt ">
</label ><br > < input type =" submit " name =" subm it " value =" Subm it " > <inpu t type = reset >
</form >
COMP284 Scripting Languages Lecture 5 Slide L5 – 23
Functions Variable-length argument lists
Functions with variable number of arguments
The number of arguments in a function call is allowed to exceed the
number of its parameters
; the parameter list only specifies the minimum number of arguments
•
int func_num_args()
returns the number of arguments passed to a function
•
mixed func_get_arg(arg_num)
returns the specified argument, or FALSE on error
•
array func_get_args()
returns an array with copies of the arguments passed to a function
function sum () { // no minimum numbe r of arguments
if ( func_num _ a r gs () < 1) return null ;
$sum = 0;
foreac h ( func_get _ a r gs () as $value ) { $sum += $ value ; }
return $ sum ;
}
COMP284 Scripting Languages Lecture 5 Slide L5 – 24
PHP libraries Include/Require
Including and requiring files
•
It is often convenient to build up libraries of function definitions,
stored in one or more files, that are then reused in PHP scripts
•
PHP provides the commands include, include_once, require, and
require_once to incorporate the content of a file into a PHP script
includ e ' mylibrary . php ';
•
PHP code in a library file must be enclosed within a PHP start tag
<?php and an end PHP tag ?>
•
The incorporated content inherits the scope of the line in which an
include command occurs
•
If no absolute or relative path is specified, PHP will search for the file
•
first, in the directories in the include path include_path
•
second, in the script’s directory
•
third, in the current working directory
COMP284 Scripting Languages Lecture 5 Slide L5 – 25
PHP libraries Include/Require
Including and requiring files
•
Several include or require commands for the same library file
results in the file being incorporated several times
; defining a function more than once results in an error
•
Several include_once or require_once commands for the same
library file results in the file being incorporated only once
•
If a library file requested by include and include_once cannot be
found, PHP generates a warning but continues the execution of the
requesting script
•
If a library file requested by require and require_once cannot be
found, PHP generates a error and stops execution of the requesting
script
COMP284 Scripting Languages Lecture 5 Slide L5 – 26
PHP libraries Include/Require
PHP Libraries: Example
mylibrary.php
<? php
function bubble_sort ( $ array ) {
... swap ( $array , $j , $j +1); ...
return $array ;
}
function swap (& $array , $i , $j ) {
...
}
?>
example.php
<? php
require_onc e ' mylibrary . php ';
$array = array (2 ,4 ,3 ,9 ,6 ,8 ,5 ,1);
$sorted = bubble_sort ( $ array );
?>
COMP284 Scripting Languages Lecture 5 Slide L5 – 27
PHP libraries Include/Require
Revision
•
Read
•
Chapter 4: Expressions and Control Flow in PHP
•
Chapter 5: PHP Functions and Objects
of R. Nixon: Learning PHP, MySQL, and JavaScript:
with jQuery, CSS & HTML5. O’Reilly, 2018.
•
Read
•
http://uk.php.net/manual/en/language.control-structures.php
•
http://uk.php.net/manual/en/language.functions.php
•
http://uk.php.net/manual/en/function.include.php
•
http://uk.php.net/manual/en/function.include-once.php
•
http://uk.php.net/manual/en/function.require.php
•
http://uk.php.net/manual/en/function.require-once.php
of P. Cowburn (ed.): PHP Manual. The PHP Group, 24 Dec 2019.
http://uk.php.net/manual/en [accessed 25 Dec 2019]
COMP284 Scripting Languages Lecture 5 Slide L5 – 28
