I took the answer BoltClock provided and tweaked it a bit with ease of configuration in mind.
// Shortens a number and attaches K, M, B, etc. accordingly
function number_shorten($number, $precision = 3, $divisors = null) {
    // Setup default $divisors if not provided
    if (!isset($divisors)) {
        $divisors = array(
            pow(1000, 0) => '', // 1000^0 == 1
            pow(1000, 1) => 'K', // Thousand
            pow(1000, 2) => 'M', // Million
            pow(1000, 3) => 'B', // Billion
            pow(1000, 4) => 'T', // Trillion
            pow(1000, 5) => 'Qa', // Quadrillion
            pow(1000, 6) => 'Qi', // Quintillion
        );    
    }
    // Loop through each $divisor and find the
    // lowest amount that matches
    foreach ($divisors as $divisor => $shorthand) {
        if (abs($number) < ($divisor * 1000)) {
            // We found a match!
            break;
        }
    }
    // We found our match, or there were no matches.
    // Either way, use the last defined value for $divisor.
    return number_format($number / $divisor, $precision) . $shorthand;
}