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function_exists

(PHP 3 >= 3.0.7, PHP 4, PHP 5)

function_exists -- Return TRUE if the given function has been defined

说明

bool function_exists ( string function_name )

Checks the list of defined functions, both built-in (internal) and user-defined, for function_name.

参数

function_name

The function name, as a string.

返回值

Returns TRUE if function_name exists and is a function, FALSE otherwise.

注意: This function will return FALSE for constructs, such as include_once() and echo().

范例

例 1. function_exists() example

<?php
if (function_exists('imap_open')) {
    echo "IMAP functions are available.<br />\n";
} else {
    echo "IMAP functions are not available.<br />\n";
}
?>

注释

注意: A function name may exist even if the function itself is unusable due to configuration or compiling options (with the image functions being an example).

参见

method_exists()
is_callable()
get_defined_functions()



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