NEVER USE this UUIDv4 generator

genuuid4.php

#!/usr/bin/env php
<?php
/**
 * Try: bin/genuuid4.php 65536 | sort | uniq -d
 */
class UuidV4
{
    const VERSION = 4;
    const VARIANT = 2;

    /**
     * @return string
     */
    public static function generate(): string
    {
        // 128 bit random bytes sequence by ASCII string
        $bytes = static::random16Bytes();

        // Text formatted UUIDv4
        // random 122 bits (6 bit reserved)
        return static::fromBytes($bytes);
    }

    protected static function fromBytes(string $bytes): string
    {
        assert(strlen($bytes) >= 16);
        return
            // 32 bit random
            bin2hex(substr($bytes, 0, 4)) .
            '-' .
            // 16 bit random
            bin2hex(substr($bytes, 4, 2)) .
            '-' .
            // 4 bit version + 12 bit random
            base_convert(
                (self::VERSION << 4) | 0b00001111 & ord(substr($bytes, 6, 1)),
                10,
                16
            ) . bin2hex(substr($bytes, 7, 1)) .
            '-' .
            // 2 bit variant + 14 bit random
            base_convert(
                (self::VARIANT << 6) | 0x00111111 & ord(substr($bytes, 8, 1)),
                10,
                16
            ) . bin2hex(substr($bytes, 9, 1)) .
            '-' .
            // 48 bit random
            bin2hex(substr($bytes, 10, 6));
    }

    protected static function random16Bytes(): string
    {
        // 16 bit * 8 --> 16 bytes
        $bytes = "";
        for ($i = 0; $i < 8; $i++) {
            $v = mt_rand(0, 0xffff);
            $bytes .= chr($v & 0xff00 >> 8);
            $bytes .= chr($v & 0x00ff);
        }
        return $bytes;
    }
}

$repeats = $argc > 1 ? intval($argv[1]) : 1;
for ($i = 0; $i < $repeats; $i++) {
    srand(random_int(0, PHP_INT_MAX));
    echo UuidV4::generate() . "\n";
}