Return TRUE if the given function has been defined (PHP 3 >= 3.0.7, PHP 4, PHP 5)
bool function_exists ( string function_name )

Checks the list of defined functions, both built-in (internal) and user-defined, for function_name. 如果成功则返回 TRUE,失败则返回 FALSE

if (function_exists('imap_open')) {
"IMAP functions are available.<br />\n";
} else {
"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(), is_callable() and get_defined_functions().