<?php



function randomChoose(array $array)

{

    $randomChance = mt_rand(0, array_sum($array));

    $chances = 0;

    foreach ($array as $value => $chance) {

        $chances += $chance;

        if ($randomChance <= $chances) {

            return $value;

        }

    }

}



var_dump(randomChoose([

    'а' => 10,

    'б' => 15,

    'в' => 25,

    'г' => 50,

]));