Three small examples of PHP determining whether constants, variables and functions exist

/* Determine whether the constant exists*/
if (defined('MYCONSTANT')) {
echo MYCONSTANT;
}
 //Judge whether the variable exists
if (isset($myvar)) {
echo "The variable $myvar exists.";
}
//Judge whether the function exists
if (function_exists(' imap_open')) {
echo "Function imag_openn exists";
} else {
echo "Function imag_open does not existn";
}
?>