Note: str_shuffle() internally uses rand(), which is unsuitable for cryptography purposes (e.g. generating random passwords). You want a secure random number generator instead. It also doesn't allow characters to repeat.

One more way.

UPDATED (now this generates any length of string):

function generateRandomString($length = 10) {
    return substr(str_shuffle(str_repeat($x='0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', ceil($length/strlen($x)) )),1,$length);
}

echo  generateRandomString();  // OR: generateRandomString(24)

That's it. :)

There are a lot of answers to this question, but none of them leverage a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG).

The simple, secure, and correct answer is to use RandomLib and don't reinvent the wheel.

For those of you who insist on inventing your own solution, PHP 7.0.0 will provide random_int() for this purpose; if you're still on PHP 5.x, we wrote a PHP 5 polyfill for random_int() so you can use the new API even before you upgrade to PHP 7.

Safely generating random integers in PHP isn't a trivial task. You should always check with your resident StackExchange cryptography experts before you deploy a home-grown algorithm in production.

With a secure integer generator in place, generating a random string with a CSPRNG is a walk in the park.

Creating a Secure, Random String

/**
 * Generate a random string, using a cryptographically secure 
 * pseudorandom number generator (random_int)
 * 
 * For PHP 7, random_int is a PHP core function
 * For PHP 5.x, depends on https://github.com/paragonie/random_compat
 * 
 * @param int $length      How many characters do we want?
 * @param string $keyspace A string of all possible characters
 *                         to select from
 * @return string
 */
function random_str($length, $keyspace = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
{
    $str = '';
    $max = mb_strlen($keyspace, '8bit') - 1;
    for ($i = 0; $i < $length; ++$i) {
        $str .= $keyspace[random_int(0, $max)];
    }
    return $str;
}

Usage:

$a = random_str(32);
$b = random_str(8, 'abcdefghijklmnopqrstuvwxyz');

Demo: https://3v4l.org/KKlc3 (Ignore the PHP 5 failures; it needs random_compat)

@tasmaniski: your answer worked for me. I had the same problem, and I would suggest it for those who are ever looking for the same answer. Here it is from tasmaniski:

<?php 
    $random = substr( md5(rand()), 0, 7);
    echo $random;
?>

Creates a 20 char long hexdec string:

$string = bin2hex(openssl_random_pseudo_bytes(10));

In PHP 7 (random_bytes()):

$string = base64_encode(random_bytes(10)); // about 14 chars
// or
$string = bin2hex(random_bytes(10)); // 20 chars

Depending on your application (I wanted to generate passwords), you could use

$string = base64_encode(openssl_random_pseudo_bytes(30));

Being base64, they may contain = or - as well as the requested characters. You could generate a longer string, then filter and trim it to remove those.

openssl_random_pseudo_bytes seems to be the recommended way way to generate a proper random number in php. Why rand doesn't use /dev/random I don't know.

$randstring in the function scope is not the same as the scope where you call it. You have to assign the return value to a variable.

$randstring = RandomString();
echo $randstring;

Or just directly echo the return value:

echo RandomString();

Also, in your function you have a little mistake. Within the for loop, you need to use .= so each character gets appended to the string. By using = you are overwriting it with each new character instead of appending.

$randstring .= $characters[rand(0, strlen($characters))];

function generateRandomString($length = 15)
{
    return substr(sha1(rand()), 0, $length);
}

Tada!

A better way to implement this function is:

function RandomString($length) {
    $keys = array_merge(range(0,9), range('a', 'z'));

    for($i=0; $i < $length; $i++) {
        $key .= $keys[mt_rand(0, count($keys) - 1)];
    }
    return $key;
}

echo RandomString(20);

mt_rand is more random according to this and this

I know this may be a bit late to the game, but here is a simple one-liner that generates a true random string without any script level looping or use of openssl libraries.

echo substr(str_shuffle(str_repeat('0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', mt_rand(1,10))),1,10);

To break it down so the parameters are clear

// Character List to Pick from
$chrList = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

// Minimum/Maximum times to repeat character List to seed from
$chrRepeatMin = 1; // Minimum times to repeat the seed string
$chrRepeatMax = 10; // Maximum times to repeat the seed string

// Length of Random String returned
$chrRandomLength = 10;

// The ONE LINE random command with the above variables.
echo substr(str_shuffle(str_repeat($chrList, mt_rand($chrRepeatMin,$chrRepeatMax))),1,$chrRandomLength);

This method works by randomly repeating the character list, then shuffles the combined string, and returns the number of characters specified.

You can further randomize this, by randomizing the length of the returned string, replacing $chrRandomLength with mt_rand(8, 15) (for a random string between 8 and 15 characters).

First, you define the alphabet you want to use:

$alphanum = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
$special  = '~!@#$%^&*(){}[],./?';
$alphabet = $alphanum . $special;

Then, use openssl_random_pseudo_bytes() to generate proper random data:

$len = 12; // length of password
$random = openssl_random_pseudo_bytes($len);

Finally, you use this random data to create the password. Because each character in $random can be chr(0) until chr(255), the code uses the remainder after division of its ordinal value with $alphabet_length to make sure only characters from the alphabet are picked (note that doing so biases the randomness):

$alphabet_length = strlen($alphabet);
$password = '';
for ($i = 0; $i < $len; ++$i) {
    $password .= $alphabet[ord($random[$i]) % $alphabet_length];
}

Alternatively, and generally better, is to use RandomLib and SecurityLib:

use SecurityLib\Strength;

$factory = new RandomLib\Factory;
$generator = $factory->getGenerator(new Strength(Strength::MEDIUM));

$password = $generator->generateString(12, $alphabet);

function rndStr($len = 64) {
     $randomData = file_get_contents('/dev/urandom', false, null, 0, $len) . uniqid(mt_rand(), true);
     $str = substr(str_replace(array('/','=','+'),'', base64_encode($randomData)),0,$len);
    return $str;
}

The edited version of the function works fine, just one issue I found: You used the wrong character to enclose $characters, so the ’ character is sometimes part of the random string that is generated.

To fix this, change:

$characters = ’0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ’;

to:

$characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

This way only the enclosed characters are used, and the ’ character will never be a part of the random string that is generated.

One very quick way is to do something like:

substr(md5(rand()),0,10);

This will generate a random string with the length of 10 chars. Of course, some might say it's a bit more heavy on the computational side, but nowadays processors are optimized to run md5 or sha256 algorithm very quickly. And of course, if the rand() function returns the same value, the result will be the same, having a 1 / 32767 chance of being the same. If security's the issue, then just change rand() to mt_rand()

Another one-liner, which generates a random string of 10 chars with letters and numbers. It will create an array with range (adjust the second parameter to set the size), loops over this array and assigns a random ascii-char (range 0-9 or a-z), then implodes the array to get a string.

$str = implode('', array_map(function () { return chr(rand(0, 1) ? rand(48, 57) : rand(97, 122)); }, range(0, 9)));

Note: only works in PHP 5.3+

Helper function from Laravel 5 framework

/**
 * Generate a "random" alpha-numeric string.
 *
 * Should not be considered sufficient for cryptography, etc.
 *
 * @param  int  $length
 * @return string
 */
function str_random($length = 16)
{
    $pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

    return substr(str_shuffle(str_repeat($pool, $length)), 0, $length);
}

I liked the last comment which used openssl_random_pseudo_bytes, but it wasn't a solution for me as I still had to remove the characters I didn't want, and I wasn't able to get a set length string. Here is my solution...

function rndStr($len = 20) {
    $rnd='';
    for($i=0;$i<$len;$i++) {
        do {
            $byte = openssl_random_pseudo_bytes(1);
            $asc = chr(base_convert(substr(bin2hex($byte),0,2),16,10));
        } while(!ctype_alnum($asc));
        $rnd .= $asc;
    }
    return $rnd;
}

one line.
fast for huge strings with some uniqueness.

function random_string($length){
    return substr(str_repeat(md5(rand()), ceil($length/32)), 0, $length);
}

This one was taken from adminer sources:

/** Get a random string
* @return string 32 hexadecimal characters
*/
function rand_string() {
    return md5(uniqid(mt_rand(), true));
}

Adminer, database management tool written in PHP.

function randomString() {
       return md5(rand(100, 200));
}

Another way to generate a random string in PHP is:

function RandomString($length) {
    $original_string = array_merge(range(0,9), range('a','z'), range('A', 'Z'));
    $original_string = implode("", $original_string);
    return substr(str_shuffle($original_string), 0, $length);
}
echo RandomString(6);

Here is how I am doing it to get a true unique random key:

$Length = 10;
$RandomString = substr(str_shuffle(md5(time())), 0, $Length);
echo $RandomString;

You can use time() since it is a Unix timestamp and is always unique compared to other random mentioned above. You can then generate the md5sum of that and take the desired length you need from the generated MD5 string. In this case I am using 10 characters, and I could use a longer string if I would want to make it more unique.

I hope this helps.

There is a simple code:

echo implode("",array_map(create_function('$s','return substr($s,mt_rand(0,strlen($s)),1);'),array_fill(0,16,"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")));

There is simple guide:

• To change the length of string, please change the 16 to another value, only.
• To to select from different characters, please change the character string.

<?php
    /**
     * Creates a random string
     *
     * @param (int) $length
     *   Length in characters
     * @param (array) $ranges
     *   (optional) Array of ranges to be used
     *
     * @return
     * Random string
    */
    function random_string($length, $ranges = array('0-9', 'a-z', 'A-Z')) {
        foreach ($ranges as $r) $s .= implode(range(array_shift($r = explode('-', $r)), $r[1]));
        while (strlen($s) < $length) $s .= $s;
        return substr(str_shuffle($s), 0, $length);
    }

    // Examples:
    $l = 100;
    echo '<b>Default:</b> ' . random_string($l) . '<br />';
    echo '<b>Lower Case only:</b> ' . random_string($l, array('a-z')) . '<br />';
    echo '<b>HEX only:</b> ' . random_string($l, array('0-9', 'A-F')) . '<br />';
    echo '<b>BIN only:</b> ' . random_string($l, array('0-1')) . '<br />';

/* End of file */

function getRandomString($length) {
  $salt = array_merge(range('a', 'z'), range(0, 9));
  $maxIndex = count($salt) - 1;

  $result = '';
  for ($i = 0; $i < $length; $i++) {
    $index = mt_rand(0, $maxIndex);
    $result .= $salt[$index];
  }
  return $result
}

Source: PHP Function that Generates Random Characters

This PHP function worked for me:

function cvf_ps_generate_random_code($length=10) {

   $string = '';
   // You can define your own characters here.
   $characters = "23456789ABCDEFHJKLMNPRTVWXYZabcdefghijklmnopqrstuvwxyz";

   for ($p = 0; $p < $length; $p++) {
       $string .= $characters[mt_rand(0, strlen($characters)-1)];
   }

   return $string;

}

Usage:

echo cvf_ps_generate_random_code(5);

The following function generates pseudo string of any length.

/**
 * Returns random string of a given length.
 */
function get_random_string($length) {
  $pull = [];
  while (count($pull) < $length) {
    $pull = array_merge($pull, range(0, 9), range('a', 'z'), range('A', 'Z'));
  }
  shuffle($pull);
  return substr(implode($pull), 0, $length);
}

function randomName($length = 8) {
  $values = array_merge(range(65, 90), range(97, 122), range(48, 57));
  $max = count($values) - 1;
  $str = chr(mt_rand(97, 122));
  for ($i = 1; $i < $length; $i++) {
    $str .= chr($values[mt_rand(0, $max)]);
  }
  return $str;
}

If you are using this random string in a place where a user might see it or use it (eg as a password generator, you might want to limit the set of characters used to exclude vowels. That way you will not accidentally generate bad words and offend someone. Don't laugh it happens.

try this:

function generate_name ($length = LENGTH_IMG_PATH) {
    $image_name = "";
    $possible = "0123456789abcdefghijklmnopqrstuvwxyz";

    $i = 0;

    while ($i < $length) {

        $char = substr($possible, mt_rand(0, strlen($possible)-1), 1);

        if (!strstr($image_name, $char)) {
            $image_name .= $char;
            $i++;               
        }              
    }            
    return $image_name;
}