Shorten long numbers to K/M/B?

function number_abbr($number)
{
    $abbrevs = [12 => 'T', 9 => 'B', 6 => 'M', 3 => 'K', 0 => ''];

    foreach ($abbrevs as $exponent => $abbrev) {
        if (abs($number) >= pow(10, $exponent)) {
            $display = $number / pow(10, $exponent);
            $decimals = ($exponent >= 3 && round($display) < 100) ? 1 : 0;
            $number = number_format($display, $decimals).$abbrev;
            break;
        }
    }

    return $number;
}

Works for positives and negatives.

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;
}

Use number_format():

if ($n < 1000000) {
    // Anything less than a million
    $n_format = number_format($n);
} else if ($n < 1000000000) {
    // Anything less than a billion
    $n_format = number_format($n / 1000000, 3) . 'M';
} else {
    // At least a billion
    $n_format = number_format($n / 1000000000, 3) . 'B';
}

I would totally appreciate any custom functions to dynamically choose the limit if possible.

If "limit" refers to the number of decimal places (the precision), that's easy:

function custom_number_format($n, $precision = 3) {
    if ($n < 1000000) {
        // Anything less than a million
        $n_format = number_format($n);
    } else if ($n < 1000000000) {
        // Anything less than a billion
        $n_format = number_format($n / 1000000, $precision) . 'M';
    } else {
        // At least a billion
        $n_format = number_format($n / 1000000000, $precision) . 'B';
    }

    return $n_format;
}

<?php
// Converts a number into a short version, eg: 1000 -> 1k
// Based on: http://stackoverflow.com/a/4371114
function number_format_short( $n, $precision = 1 ) {
    if ($n < 900) {
        // 0 - 900
        $n_format = number_format($n, $precision);
        $suffix = '';
    } else if ($n < 900000) {
        // 0.9k-850k
        $n_format = number_format($n / 1000, $precision);
        $suffix = 'K';
    } else if ($n < 900000000) {
        // 0.9m-850m
        $n_format = number_format($n / 1000000, $precision);
        $suffix = 'M';
    } else if ($n < 900000000000) {
        // 0.9b-850b
        $n_format = number_format($n / 1000000000, $precision);
        $suffix = 'B';
    } else {
        // 0.9t+
        $n_format = number_format($n / 1000000000000, $precision);
        $suffix = 'T';
    }
  // Remove unecessary zeroes after decimal. "1.0" -> "1"; "1.00" -> "1"
  // Intentionally does not affect partials, eg "1.50" -> "1.50"
    if ( $precision > 0 ) {
        $dotzero = '.' . str_repeat( '0', $precision );
        $n_format = str_replace( $dotzero, '', $n_format );
    }
    return $n_format . $suffix;
}
/*
Example Usage:
number_format_short(7201); // Output: 7.2k
Demo:
echo '<table>';
for($d = 0; $d < 16; $d++ ) {
    $n = intval("09" . str_repeat( "0", $d ));
    $n = $n / 10;
    echo number_format_short($n) .'<br>'; // 0.9
    $n = intval("1" . str_repeat( "0", $d ));
    echo number_format_short($n) .'<br>'; // 1.0
    $n = intval("11" . str_repeat( "0", $d ));;
    $n = $n / 10;
    echo number_format_short($n) .'<br>'; // 1.1
}
echo '</table>';
Demo Output:
0.9
1
1.1

9
10
11

90
100
110

0.9K
1K
1.1K

9K
10K
11K

90K
100K
110K

0.9M
1M
1.1M

9M
10M
11M

90M
100M
110M

0.9B
1B
1.1B

9B
10B
11B

90B
100B
110B

0.9T
1T
1.1T

9T
10T
11T

90T
100T
110T

900T
1,000T
1,100T
*/