Firebase push algorithm in PHP (ref: https://www.firebase.com/blog/2015-02-11-firebase-unique-identifiers.html)

PushId.php

<?php

/**
 * Fancy ID generator that creates 20-character string identifiers with the following properties:
 *
 * 1. They're based on timestamp so that they sort *after* any existing ids.
 * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs.
 * 3. They sort *lexicographically* (so the timestamp is converted to characters that will sort properly).
 * 4. They're monotonically increasing.  Even if you generate more than one in the same timestamp, the
 *    latter ones will sort after the former ones.  We do this by using the previous random bits
 *    but "incrementing" them by 1 (only in the case of a timestamp collision).
 */
class PushId
{
    /**
     * Modeled after base64 web-safe chars, but ordered by ASCII.
     *
     * @var string
     */
    const PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';

    /**
     * Timestamp of last push, used to prevent local collisions if you push twice in one ms.
     *
     * @var int
     */
    private static $lastPushTime = 0;

    /**
     * We generate 72-bits of randomness which get turned into 12 characters and appended to the
     * timestamp to prevent collisions with other clients.  We store the last characters we
     * generated because in the event of a collision, we'll use those same characters except
     * "incremented" by one.
     *
     * @var array
     */
    private static $lastRandChars = [];

    /**
     * @return string
     */
    public static function generate()
    {
        $now = (int) microtime(true) * 1000;
        $isDuplicateTime = ($now === static::$lastPushTime);
        static::$lastPushTime = $now;

        $timeStampChars = new SplFixedArray(8);
        for ($i = 7; $i >= 0; $i--) {
            $timeStampChars[$i] = substr(self::PUSH_CHARS, $now % 64, 1);
            // NOTE: Can't use << here because javascript will convert to int and lose the upper bits.
            $now = (int) floor($now / 64);
        }

        static::assert($now === 0, 'We should have converted the entire timestamp.');

        $id = implode('', $timeStampChars->toArray());

        if (!$isDuplicateTime) {
            for ($i = 0; $i < 12; $i++) {
                $lastRandChars[$i] = floor(rand(0, 64));
            }
        } else {
            // If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
            for ($i = 11; $i >= 0 && static::$lastRandChars[$i] === 63; $i--) {
                static::$lastRandChars[$i] = 0;
            }
            static::$lastRandChars[$i]++;
        }
        for ($i = 0; $i < 12; $i++) {
            $id .= substr(self::PUSH_CHARS, $lastRandChars[$i], 1);
        }

        static::assert(strlen($id) === 20, 'Length should be 20.');

        return $id;
    }

    /**
     * @param bool   $condition
     * @param string $message
     */
    private static function assert($condition, $message = '')
    {
        if ($condition !== true) {
            throw new RuntimeException($message);
        }
    }
}

The issue with this implementation is that the character key is allowed to get to 64 and there is no character for 64. @jbroadway's solution doesn't quite fix it either. There was also an issue with a strict type check being the wrong data type.

Take a look at this version: https://gist.github.com/datasage/fbd4cdc725598e184c7d

I noticed that the value of $now always ended with 000. The (int) conversion takes precedence over *1000, so you never get ms precision with above solution. This is a bug specific to the PHP fork; the js implementation has ms in the timestamp.
To fix, replace this line:
$now = (int) microtime(true) * 1000;
with this:
$now = (int) (microtime(true) * 1000);

This code does not seem to work since 28/11/2017 01:21 AM (Paris)
The part of the code $now % 64 on line 50 goes into negative
This results in the error:

Fatal error: Uncaught exception 'RuntimeException' with message 'We should have converted the entire timestamp.'

My temporary solution was to add after line 54:
if($now < 0 ) $now = 0;

The 61st line sometimes causes errors, since there isn't a 64th character. It should be replaced with $lastRandChars[$i] = floor(rand(0, 63));