function_exists --
Return TRUE if the given function has been defined
Syntax
bool function_exists ( string function_name )
Checks the list of defined functions, both built-in (internal) and
user-defined, for function_name. Returns TRUE on success or FALSE on failure.
<?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().
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Php function exists syntax tutorial
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