<?php
function intToAlphaBaseN($n,$baseArray) {
    $l=count($baseArray);
    $s = '';
    for ($i = 1; $n >= 0 && $i < 10; $i++) {
        $s =  $baseArray[($n % pow($l, $i) / pow($l, $i - 1))].$s;
        $n -= pow($l, $i);
    }
    return $s;
}

$base=array_merge(range('A','Z'), range(0,9));
$zero = $base[0];

//an integer number
$r=rand(0, 999999);
echo "$r converts to :".str_pad(intToAlphaBaseN($r,$base), 6, $zero, STR_PAD_LEFT)."\n";
//an integer number
$r++;
echo "$r converts to :".str_pad(intToAlphaBaseN($r,$base), 6, $zero, STR_PAD_LEFT)."\n";
//an integer number
$r++;
echo "$r converts to :".str_pad(intToAlphaBaseN($r,$base), 6, $zero, STR_PAD_LEFT)."\n";
?>