PHP Random Number With Alphabet

<?php

function randomkey(){ return substr(str_shuffle(str_repeat("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ", 10)), 0, 6); }

for ($i=0;$i<5;++$i)
  echo randomkey()."\n";


 function random_string($length) {
    $key = '';
    $keys =             array("↑","↓","→","←","2↑","2↓","2→","2←","3↑","3→","3↓","3←");
    for ($i = 0; $i < $length; $i++) {
      $key .= $keys[array_rand($keys)];
    }
    return $key;
    }
    $rand_string = random_string(15);
    echo htmlentities($rand_string);

public function code_generator($max_num = 6, $prefix = "PGD")
    {
        if ( (strlen(self::TOTAL_MAX_LIMIT)+$max_num) > self::TOTAL_MAX_LIMIT ) {
            return false;
        } else {

            $min = pow(10, $max_num - 1) ;
            $max = pow(10, $max_num) - 1;
            return $prefix.mt_rand($min, $max);
            ///return $prefix.mt_rand($max_num,$max_num); /* return fix length of number */
        }
    }