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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

## Description

bool function_exists ( string function_name )

Checks the list of defined functions, both built-in (internal) and user-defined, for function_name. 成功回傳`TRUE`失敗回傳`FALSE`。

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

Note that 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). Also note that function_exists() will return `FALSE` for constructs, such as include_once() and echo().

See also method_exists() and get_defined_functions().

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