function_exists — Return true if the given function has been defined
Syntax:
boolean function_exists()
Parameter: This function accepts a single parameter . This is the name of function that we want to search in the list of defined function. This is a string type parameter.
Return Values: This function returns a Boolean value. In case a function with the name exists it returns TRUE, otherwise it returns FALSE. This function will also return FALSE for constructs like “include_once”, “echo” etc.