How to create a random string using PHP?

If you want to allow repetitive occurences of characters, you can use this function:

function randString($length, $charset='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789')
{
    $str = '';
    $count = strlen($charset);
    while ($length--) {
        $str .= $charset[mt_rand(0, $count-1)];
    }
    return $str;
}

The basic algorithm is to generate <length> times a random number between 0 and <number of characters> − 1 we use as index to pick a character from our set and concatenate those characters. The 0 and <number of characters> − 1 bounds represent the bounds of the $charset string as the first character is addressed with $charset[0] and the last with $charset[count($charset) - 1].

Well, you didn't clarify all the questions I asked in my comment, but I'll assume that you want a function that can take a string of "possible" characters and a length of string to return. Commented thoroughly as requested, using more variables than I would normally, for clarity:

function get_random_string($valid_chars, $length)
{
    // start with an empty random string
    $random_string = "";

    // count the number of chars in the valid chars string so we know how many choices we have
    $num_valid_chars = strlen($valid_chars);

    // repeat the steps until we've created a string of the right length
    for ($i = 0; $i < $length; $i++)
    {
        // pick a random number from 1 up to the number of valid chars
        $random_pick = mt_rand(1, $num_valid_chars);

        // take the random character out of the string of valid chars
        // subtract 1 from $random_pick because strings are indexed starting at 0, and we started picking at 1
        $random_char = $valid_chars[$random_pick-1];

        // add the randomly-chosen char onto the end of our string so far
        $random_string .= $random_char;
    }

    // return our finished random string
    return $random_string;
}

To call this function with your example data, you'd call it something like:

$original_string = 'abcdefghi';
$random_string = get_random_string($original_string, 6);

Note that this function doesn't check for uniqueness in the valid chars passed to it. For example, if you called it with a valid chars string of 'AAAB', it would be three times more likely to choose an A for each letter as a B. That could be considered a bug or a feature, depending on your needs.

function generate_random_string($name_length = 8) {
    $alpha_numeric = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    return substr(str_shuffle(str_repeat($alpha_numeric, $name_length)), 0, $name_length);
}

Updated the code as per mzhang's great suggestion in the comments below.

Joining characters at the end should be more efficient that repeated string concatenation.

Edit #1: Added option to avoid character repetition.

Edit #2: Throws exception to avoid getting into infinite loop if $norepeat is selected and $len is greater than the charset to pick from.

Edit #3: Uses array keys to store picked random characters when $norepeat is selected, as associative array key lookup is faster than linearly searching the array.

function rand_str($len, $norepeat = true)
{
    $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    $max = strlen($chars) - 1;

    if ($norepeat && len > $max + 1) {
        throw new Exception("Non repetitive random string can't be longer than charset");
    }

    $rand_chars = array();

    while ($len) {
        $picked = $chars[mt_rand(0, $max)];

        if ($norepeat) {
            if (!array_key_exists($picked, $rand_chars)) {
                $rand_chars[$picked] = true;
                $len--;
            }
        }
        else {
            $rand_chars[] = $picked;
            $len--;
        }
    }

    return implode('', $norepeat ? array_keys($rand_chars) : $rand_chars);   
}

built on top of https://stackoverflow.com/a/853898/533426

but with php 7 cryptographically secure random function and lower AND upper case alphabet

function random($length = 8){
            $valid_chars ="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

            // start with an empty random string
            $random_string = "";

            // count the number of chars in the valid chars string so we know how many choices we have
            $num_valid_chars = strlen($valid_chars);

            // repeat the steps until we've created a string of the right length
            for ($i = 0; $i < $length; $i++)
            {
                // pick a random number from 1 up to the number of valid chars
                $random_pick = random_int(1, $num_valid_chars);

                // take the random character out of the string of valid chars
                // subtract 1 from $random_pick because strings are indexed starting at 0, and we started picking at 1
                $random_char = $valid_chars[$random_pick-1];

                // add the randomly-chosen char onto the end of our string so far
                $random_string .= $random_char;
            }

            // return our finished random string
            return $random_string;
    }

//example output XjdXHakZ, yBG8hpZG, L6jg4FpK

Do you want to create your password by a random permutation of the original letters? Should it just contain unique characters?

Use rand to choose random letters by index.

This is an old question but I want try to post my solution... I always use this my function to generate a custom random alphanumeric string...

<?php
  function random_alphanumeric($length) {
    $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ12345689';
    $my_string = '';
    for ($i = 0; $i < $length; $i++) {
      $pos = mt_rand(0, strlen($chars) -1);
      $my_string .= substr($chars, $pos, 1);
    }
    return $my_string;
  }
  $test = random_alphanumeric(50); // 50 characters
  echo $test;
?>

test: UFOruSSTCPIqxTRIIMTRkqjOGidcVlhYaS9gtwttxglheVugFM

if you need two or more unique strings you can use this trick...

$string_1 = random_alphanumeric(50);
$string_2 = random_alphanumeric(50);
while ($string_1 == $string_2) {
  $string_1 = random_alphanumeric(50);
  $string_2 = random_alphanumeric(50);
  if ($string_1 != $string_2) {
     break;
  }
}
echo $string_1;
echo "<br>\n";
echo $string_2;

$string_1: tMYicqLCHEvENwYbMUUVGTfkROxKIekEB2YXx5FHyVByp3mlJO

$string_2: XdMNJYpMlFRKFDlF6GhVn6jsBVNQ1BCCevj8yK2niFOgpDI2MU

I hope this help.

echo substr(bin2hex(random_bytes(14)), 0, $length);

this code gets a random bytes, that are converted from binary to hexadecimal, and then takes a substring of this hexadecimal string, as long you puts in $length variable

Try this

Simple enough!

function RandomFromCharset($charset,$length) 
{

$characters = $charset; // your existing charset / defined string
    $charactersLength = strlen($characters);
    $random_from_charset = '';
    for ($i = 0; $i < $length; $i++) 
    {
        $random_from_charset.= $characters[rand(0, $charactersLength - 1)];
    }

return random_from_charset;
}

Call the function as follows

RandomFromCharset($charset,$length);

where $length will be length of random string you want (this can be predefined also in the function as RandomFromCharset(charset,$length=10) ) to generate and $charset will be your existing string to which you want to restrict the characters.

One approach is to generate an md5 from a random number and extract the number of characters you want:

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

mt_rand will generate a random number, md5 will create a 32 character string (containing both letters and numbers) and, in this example, we're extracting the first 7 characters of text.

you could make an array of characters then use rand() to pick a letter from the array and added it to a string.

$letters = array( [0] => 'a' [1] => 'b' [2] => 'c' [3] => 'd' ... [25] = 'z');

$lengthOfString = 10;
$str = '';

while( $lengthOfString-- )
{
   $str .= $letters[rand(0,25)];
}
echo $str;

*note that this does allow repeat characters