Generate incremental id of numbers, Upper and Lower letters

This should work for you:

<?php

    $hello = "aaa";

    //'aaa0' -> 'aaa9'
    for ($count = 0; $count <= 9; $count++)  
        echo $hello . $count . "<br />";
    //'aaaa' -> 'aaaz'
    foreach (range('a', 'z') as $char)
        echo $hello . $char . "<br />";
    //'aaaA' -> 'aaaZ'
    foreach (range('A', 'Z') as $char)
        echo $hello . $char . "<br />";

?>

EDIT:

This works only with 3 digit's. After you run out of memory for sure.

<?php

    $array = array();
    $maxLength = 3;
    $output = array();

    ini_set('memory_limit', '-1');
    $time_start = microtime(true);

    foreach(range(0, 9) as $number)
        $array[] = $number;
    foreach(range('a', 'z') as $char)
        $array[] = $char;
    foreach(range('A', 'Z') as $char)
        $array[] = $char;


    function everyCombination($array, $arrLength, $size, $perArr = array(), $pos = 0, &$found = array()) {
        if ($size == $pos) {
            $found[] = vsprintf("%s%s%s", $perArr);
            return;
        }

        for ($count = 0; $count < $arrLength; $count++) {
            $perArr[$pos] = $array[$count]; 
            everyCombination($array, $arrLength, $size, $perArr, $pos+1, $found); 
        }
        return $found;
    }

    $output = everyCombination($array, count($array), $maxLength);

    for($count = 0; $count < count($output); $count++)
        echo $output[$count] . "<br/>";

    echo "DONE!";
    $time_end = microtime(true);
    $time = $time_end - $time_start;
    echo  round($time,2) . " s";

?>

Using 0,1-9, a-z and A-Z is "Base 62". Converting from base 10, to base 62 is very easy in PHP.

<?php
echo base_convert(10123, 10,26), "\n";
// outputs: 'ep9'
echo base_convert('ep9', 26, 10), "\n";
// output 10123