Split an array into chunks based on the first letter of one of its column values.

ChunkAlphabetically.php

<?php

global $alpha_chunks;

# Initialize array
$alpha_chunks = array();

function chunkNames(&$value, $key, $letter) {
    global $alpha_chunks;

    /* 
    If the first letter of the last name == $letter, Add the entry to a multi-dimensional array with each
    $letter is the key. Change 'lastname' to match the name of that column. 
    */
    if ($value['lastname'][0] === $letter ) {
        $alpha_chunks[$letter][$key] = $value;
    } 
}

# Can be any array of characters. 
$atoz = range('A','Z'); 

foreach($atoz as $letter) { 
    # $array_to_walk isn't defined here, but this is should be the array you'd like to split. 
    array_walk($array_to_walk, 'chunkNames', $letter);
}