PHP defined()

defined(x) function checks whether a constant defined or not. It returns TRUE if defined, or FALSE if not. if x is a variable, return FALSE.

<?PHP
	define("MAX",10000);
	if (defined("MAX")) echo "defined";  //defined
	if (defined("M_PI")) echo "defined";  //defined
	
	$max = 100;
    if (defined("max")) echo "defined";
	else echo "not defined";	//not defined
?>

The parameter must be quoted, otherwise FALSE will be returned.

<?PHP
	define("MAX",10000);
	if (defined(MAX)) echo "defined"; 
	else echo "not defined";  //not defined
	
	if (defined(M_PI)) echo "defined"; 
	else echo "not defined";  //not defined	
?>

To check whether a variable is declared or not, use function isset(). It will return TRUE if declared, otherwise FALSE. If a variable is declared as NULL, it will return FALSE too.

<?PHP
	$max = 100;
    if (isset($max)) echo "declared"; //declared
	else echo "not declared";  

	$max = NULL;
    if (isset($max)) echo "declared";
	else echo "not declared";  //not declared
?>

To check whether a function has been defined or not, use function function_exists(). It will return TRUE if defined, otherwise FALSE. The parameter should be quoted like defined().

<?PHP
    if (function_exists("preg_split")) echo "defined"; //defined
	else echo "not defined";
	
	function uppercasestr($str)
	{
		$str = strtolower($str);
		$elem = explode(" ",$str);
		$str = "";
		foreach($elem as $word)
		{
		   if ($str == "") $str .= ucfirst($word);
		   else $str .= " " . ucfirst($word);
		}
		return $str;
	}
	if (function_exists("uppercasestr")) echo "defined"; //defined
	else echo "not defined";
?>

Some PHP predefined constants are listed below. For more PHP predefined constants, please check constants.