Manual do PHP
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function_exists

(PHP 3>= 3.0.7, PHP 4 )

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. Retorna TRUE em caso de sucesso ou FALSE em falhas. 

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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func_num_argsAcimaget_defined_functions