Function name must be a string error

Hello again; I have solved this before. But it has been too long ago. I don't remember what the fix was; file with function definition: $_funct = function(/*args*/){ // code} class def: private static $_proc = ''; __construct() requires file with $_funct definition and assigns it to self::$_proc require(/*file with funct definition*/) self::$_proc = $_funct; later in $_POST processing the function is called: self::$_proc(/*args*/) // -> Function name must be a string There are actually more steps to this, but it would require a lot of code reproduced here for it to make sense. This abstraction is about as simple as I can represent it. In the processing the self::$_proc function is being called under different circumstances. The error occurs on the first time it is called. So It appears that it is not a case where one particular line has a problem and the others don't. This has been working elsewhere in testing in the same project. I google the error and didn't get listings for the same type of issue. Thanks for time and attention; Jeff K