These are the function with no defined function. Mostly used for callback.
These are accessed by the variables on which they are defined or invoked dynamically.
$yourVariable = function(){return 1+1};
//now the variable has 2 as value
$areaOfCircle = function($a){ return 3.14*$a*$a;};
$yourOtherVariable = $areaOfCircle(7);
//now the variable has 153.86 as value