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(), print(), unset(), isset(), empty(), include(), require() and the like. You need to use your own wrapper function to utilize any of these constructs as variable functions.
Example 16-13. Variable function example
You can also call an object's method by using the variable functions feature.