PHP Tutorials: isset (), Unnset (), Empty ()

Isset (), Unnset (), Empty () is a language structure rather than a function, so it cannot be invoked by a variable function .

isset (), empty () only detects variables, and detecting anything that is not variable will result in parsing errors. The following statement is an error and will not work: empty (Addslashes ($name)). To detect if a constant is set, use the defined () function. Also note that a null byte ("\0″") is not equivalent to PHP's null constant. So Isset (NULL) can cause syntax errors.

Like Echo (), print (), include (), require () are language constructs rather than functions.