How to get a random string in php?

This tutorial includes several methods of generating a unique, random, alpha-numeric string with PHP.

1. Code:

    <?php 
	function getRandomString($length){
		$characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
		$charactersLength = strlen($characters);
		$randomString = '';
		for ($i = 0; $i < $length; $i++) {
			$randomString .= $characters[rand(0, $charactersLength - 1)];
		}
		return $randomString;
	}
        $str1 = getRandomString(20);
	echo $str1;
	echo "<br>";
	$str1 = getRandomString(18);
	echo $str1;
    ?> 

2. Example:

Input:

getRandomString(20);
getRandomString(18);

Output:

0XjUEjHU4ZH2PFSHBdyv
Xoe5b4t2m6dTcQWAY7