PHP手册 - 控制结构

PHP手册 - N: 控制结构

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wintermute (29-Aug-2007 08:45)

Sinured: You can do the same thing with logical OR; if the first test is true, the second will never be executed. <?PHP if (empty($user_id) || in_array($user_id, $banned_list)) { exit(); } ?>

Sinured (01-Aug-2007 07:59)

As mentioned below, PHP stops evaluating expressions as soon as the result is clear. So a nice shortcut for if-statements is logical AND -- if the left expression is false, then the right expression can’t possibly change the result anymore, so it’s not executed. <?php /* defines MYAPP_DIR if not already defined */ if (!defined('MYAPP_DIR')) { define('MYAPP_DIR', dirname(getcwd())); } /* the same */ !defined('MYAPP_DIR') && define('MYAPP_DIR', dirname(getcwd())); ?>

dougnoel (06-May-2006 02:29)

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.

niels dot laukens at tijd dot com (26-Dec-2004 03:49)

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 ?>

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