PHP supports the concept of variable functions. This means that if a variable name has parentheses appended to it, PHP will look for a function with the same name as whatever the variable evaluates to, and will attempt to execute it. Among other things, this can be used to implement callbacks, function tables, and so forth.
Variable functions won't work with language constructs such as echo(), unset(), isset(), empty() and include(). Although, the construct print() is an exception and will work. This is one of the major differences between PHP functions and language constructs.