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4.9 Checking whether a function is available: function_exists()

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bool function_exists ( string function_name)

If you're working with functions that are not part of the core of PHP, that is, functions that are from an extension that needs to be enabled by users, it's a smart move to use the function_exists() function. This takes a function name as its only parameter, and returns true if that function (either built-in, or one you've defined yourself) is available for use. Note that it only checks whether the function is available, not whether it will work - your system may not be configured properly for some functions.

Author's Note: If you ever want to know whether you have a function available to you, use the function_exists() function. This takes one string parameter that is the name of a function, and returns true if the function exists or false if it does not. Many people use function_exists() to find out whether they have an extension available, by calling function_exists() on a function of that extension. However, this is accomplished much more easily with the function extension_loaded() function covered later.

<< 4.8.7 The regular expressions coach		4.10 Extension functions: get_loaded_extensions(), get_extension_funcs(), dl(), and extension_loaded() >>
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deathgod - 08 Sep 2008

I have read over and over people asking the same damn thing on every single page. If you want an example when Rock Hudson thinks its not necessary then write it yourself and post it here. For Example,:) here is an example relavant to this page:

<?php
$var = function_exists(tep_my_function());

if ($var=1) echo "Wooh dude, cool function";
else echo "Dude, where\'s my function?";
?>

DoctoR JackaL - 08 Sep 2008

It would be best if you give some examples , and showing the outputs if TRUE or FALSE , thx , great book !

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