random_int

Here is a simple backporting function, it works for PHP >= 5.1

<?php
if (!function_exists('random_int')) {
    function random_int($min, $max) {
        if (!function_exists('mcrypt_create_iv')) {
            trigger_error(
                'mcrypt must be loaded for random_int to work', 
                E_USER_WARNING
            );
            return null;
        }
        
        if (!is_int($min) || !is_int($max)) {
            trigger_error('$min and $max must be integer values', E_USER_NOTICE);
            $min = (int)$min;
            $max = (int)$max;
        }
        
        if ($min > $max) {
            trigger_error('$max can\'t be lesser than $min', E_USER_WARNING);
            return null;
        }
        
        $range = $counter = $max - $min;
        $bits = 1;
        
        while ($counter >>= 1) {
            ++$bits;
        }
        
        $bytes = (int)max(ceil($bits/8), 1);
        $bitmask = pow(2, $bits) - 1;
 
        if ($bitmask >= PHP_INT_MAX) {
            $bitmask = PHP_INT_MAX;
        }
 
        do {
            $result = hexdec(
                bin2hex(
                    mcrypt_create_iv($bytes, MCRYPT_DEV_URANDOM)
                )
            ) & $bitmask;
        } while ($result > $range);
 
        return $result + $min;
    }
}
?>

Randomness test

<?php
$max = 100; // number of random values
$test = 1000000;

$array = array_fill(0, $max, 0);

for ($i = 0; $i < $test; ++$i) {
    ++$array[random_int(0, $max-1)];
}

function arrayFormatResult(&$item) {
    global $test, $max; // try to avoid this nowdays ;)
    
    $perc = ($item/($test/$max))-1;
    $item .= ' '. number_format($perc, 4, '.', '') .'%';
}

array_walk($array, 'arrayFormatResult');

print_r($array);
?>

On my system (5th gen i7) random_int() takes about 14x longer to do the same number of calculations than rand(). 

for ($i=0; $i<10000000; $i++)
        $x = rand(1, PHP_INT_MAX);

Takes 0.86 seconds for rand(), 12.29 seconds for random_int(). So use random_int() somewhat sparingly. 

If it's not really important to be truly random, use rand() instead, especially in a tight loop.

<?php 
    // PHP >= 7
    function str_rand(int $length = 16){
        $x = '';
        for($i = 1; $i <= $length; $i++){
            $x .= dechex(random_int(0,255));
        }
        return substr($x, 0, $length);
    }
    var_dump(str_rand());
?>

A small function for generating version 4 GUIDs.

<?php
function v4uuid()
{
    $a = str_pad(dechex(random_int(0x0000_0000, 0xffff_ffff)), '0', STR_PAD_LEFT);
    $b = str_pad(dechex(random_int(0x0000, 0xffff)), '0', STR_PAD_LEFT);
    $c = dechex(random_int(0x4000, 0x4fff));
    $d = dechex(random_int(0x8000, 0xbfff));
    $e = str_pad(dechex(random_int(0x0000_0000_0000, 0xffff_ffff_ffff)), '0', STR_PAD_LEFT);
    return "$a-$b-$c-$d-$e";
}
?>