PRNG Benchmarks (derived from the following code: https://gist.github.com/sarciszewski/f7bd4c0358a44321787b)

benchmark.php

<?php
function timer(callable $block) {
    $start = microtime(true);

    for ($i = 0; $i < 100000; ++$i) {
        $block();
    }

    $end = microtime(true);

    return $end - $start;
}

$buf = '';

$tests = [
    'random_bytes' => timer(function() {
        $buf = random_bytes(32);
    }),
    'openssl_random_pseudo_bytes' => timer(function() {
        $buf = openssl_random_pseudo_bytes(32);
    }),
    'mcrypt_create_iv' => timer(function() {
        $buf = mcrypt_create_iv(32, MCRYPT_DEV_URANDOM);
    }),
    'mt_rand' => timer(function() {
        $buf = '';
        for ($i = 0; $i < 32; ++$i) {
           $buf .= chr(mt_rand(0, 255));
        }
    })
];

var_dump($tests);

result.txt

array(4) {
  ["random_bytes"]=>
  float(0.038606882095337)
  ["openssl_random_pseudo_bytes"]=>
  float(0.26424193382263)
  ["mcrypt_create_iv"]=>
  float(0.42767190933228)
  ["mt_rand"]=>
  float(0.79697513580322)
}