Глава 16. Control Structures

Содержание
if
else
elseif
Alternative syntax for control structures
while
do-while
for
foreach
break
continue
switch
declare
return
require()
include()
require_once()
include_once()

Any PHP script is built out of a series of statements. A statement can be an assignment, a function call, a loop, a conditional statement of even a statement that does nothing (an empty statement). Statements usually end with a semicolon. In addition, statements can be grouped into a statement-group by encapsulating a group of statements with curly braces. A statement-group is a statement by itself as well. The various statement types are described in this chapter.

`if`

The if construct is one of the most important features of many languages, PHP included. It allows for conditional execution of code fragments. PHP features an if structure that is similar to that of C:

if (expr)
    statement

As described in the section about expressions, expression is evaluated to its Boolean value. If expression evaluates to TRUE, PHP will execute statement, and if it evaluates to FALSE - it'll ignore it. More information about what values evaluate to FALSE can be found in the 'Converting to boolean' section.

The following example would display a is bigger than b if $a is bigger than $b:


<?php

if ($a > $b)

    echo "a is bigger than b";

?>

Often you'd want to have more than one statement to be executed conditionally. Of course, there's no need to wrap each statement with an if clause. Instead, you can group several statements into a statement group. For example, this code would display a is bigger than b if $a is bigger than $b, and would then assign the value of $a into $b:


<?php

if ($a > $b) {

    echo "a is bigger than b";

    $b = $a;

}

?>

If statements can be nested indefinitely within other if statements, which provides you with complete flexibility for conditional execution of the various parts of your program.

Control Structures

28-May-2006 01:07


$foo=false will never work as it is tha assign operator.



$foo == false is required

abu_hurayrah at hidayahonline dot org
24-May-2006 01:42


Regarding lamfeust's example:



    Just a note about initializing variable



    <?php



    if($foo=false)

      print("true");

    else

      print("false");

 

    // this print on screen false



    ?>



This will print out "false" due to the expression "$foo=false" returning the value of the assignment - boolean false, in this case.



I'm not sure what, exactly, that had to do with initialization.

dougnoel
05-May-2006 06:29


Further response to Niels:



It's not laziness, it's optimization.  It saves CPUs cycles.  However, it's good to know, as it allows you to optimize your code when writing.  For example, when determining if someone has permissions to delete an object, you can do something like the following:



if ($is_admin && $has_delete_permissions)



If only an admin can have those permissions, there's no need to check for the permissions if the user is not an admin.

roan dot kattouw at home dot nl
09-Dec-2005 04:49


In response to Niels: The ANSI C standard also dictates this laziness .

niels dot laukens at tijd dot com
26-Dec-2004 07:49


For the people that know C: php is lazy when evaluating expressions. That is, as soon as it knows the outcome, it'll stop processing.



<?php

if ( FALSE && some_function() )

    echo "something";

// some_function() will not be called, since php knows that it will never have to execute the if-block

?>



This comes in nice in situations like this:

<?php

if ( file_exists($filename) && filemtime($filename) > time() )

    do_something();

// filemtime will never give an file-not-found-error, since php will stop parsing as soon as file_exists returns FALSE

?>