Function works inconsistently

I am new to PHP. I have a function that works (the value printed to the screen from within the function is exactly what I expect), but only sometimes returns the answer (other times it returns NULL). I believe I have isolated the error to something involving my use of the static feature of PHP, but I am not sure how exactly I'm transgressing/how to fix it. I tried to work around it by creating a new non static variable to store the result, and that improved my code from always returning NULL to only sometimes returning NULL. The erring function is part of a larger suite of programs I wrote, so I'll include it and the testing function I use to check that it works.

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probGen.php

<?
    require_once("randX.php");
    require_once("../displayArray.php");
    error_reporting(E_ERROR | E_WARNING | E_PARSE);

    function probGen(array $arr, float $control = 0.01) 
/*
*   Generates a valid, random probability distribution for a given array of elements, that can be used in conjunction with "probSelect()".
*   Input: 
        $arr: An array of elements.
        $control: A value that decides how much mass is allowed to be unilaterally dumped onto one element. A high value would permit distributions where most of the mass is concentrated on one element. 
        If an invalid value is provided, the default is used.
*   Output: An associative array where the keys are the elements in the original array, and the values are their probabilities. 
*/
    {
        $control = ($control <= 1 && $control >= 0)?($control):(0.01);  #Use the default value if an invalid number is supplied.
        static $result = [];    #Initialises $result with an empty array on first function call.
        static $max = 1;    #Initialises $max with 1 on first function call.
        foreach ($arr as $value) 
        {
            $x = randX(0, $max);    #Random probability value.
            $result[$value] = ($result[$value] + $x)??0;    #Initialise the array with 0 on first call, and on subsequent calls increment by $x to assign probability mass.
            $max -= $x;     #Ensures that the probability never sums to more than one.
        }
        print("<br>sum = ".array_sum($result)."<br><br>");
        displayArray($result);
/*
*   After the execution of the above code, there would be some leftover probability mass. 
*   The code below adds it to a random element.
*/
        $var = array_values($arr);
        if($max <= $control)    #To limit concentration of most of the probability mass in one variable.
        {
            $result[$var[rand(0,(count($var)-1))]] += $max; #Selects a random key and adds $max to it.
            displayArray($result);
            print("<br>sum = ".array_sum($result)."<br><br>");  
            $sol = $result;
            return $sol;
        }
        else
            probGen($arr, $control);    
    }
?>

test.php

<?

    /*
    *   This file contains some functions that can be used for assigning probabilities to an array of elements or selecting an element from said array with a given probability.
    */

    require_once("../displayArray.php");
    require_once("confirm.php");
    require_once("randX.php");
    require_once("probGen.php");
    require_once("probSelect.php");

    $test = ["California" => 0.37, "Florida" => 0.13, "New York" => 0.28, "Washington" => 0.22];
    $testX = array_keys($test);

    var_dump(probGen($testX));

    /*for ($i=0; $i < 100 ; $i++) 
    { 
        print(var_dump(confirm(probGen($testX))));
    }*/

?>

Screenshot of probGen() working as intended:

Screenshot of probGen() failing:

From testing I've been able to determine that probGen() fails if the recursion occurs more than once.