How to generate a random, unique, alphanumeric string in PHP

There are many ways to generate a random, unique, alphanumeric string in PHP which are given below:

Using str_shuffle() Function: The str_shuffle() function is an inbuilt function in PHP and is used to randomly shuffle all the characters of a string passed to the function as a parameter. When a number is passed, it treats the number as the string and shuffles it. This function does not make any change in the original string or the number passed to it as a parameter. Instead, it returns a new string which is one of the possible permutations of the string passed to it in the parameter.
Example:

PHP

<?php

// This function will return a random
// string of specified length
function random_strings($length_of_string)
{

    // String of all alphanumeric character
    $str_result = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';

    // Shuffle the $str_result and returns substring
    // of specified length
    return substr(str_shuffle($str_result), 
                       0, $length_of_string);
}

// This function will generate
// Random string of length 10
echo random_strings(10);

echo "\n";

// This function will generate
// Random string of length 8
echo random_strings(8);

?>

Output

hnQVgxd4FE
6EsbCc53

Using md5() Function: The md5() function is used to calculate the MD5 hash of a string. Pass timestamp as a argument and md5 function will convert them into 32 bit characters
Example:

PHP

<?php

// This function will return a random
// string of specified length
function random_strings($length_of_string) {
    
    // md5 the timestamps and returns substring
    // of specified length
    return substr(md5(time()), 0, $length_of_string);
}

// This function will generate 
// Random string of length 10
echo random_strings(10);

echo "\n";

// This function will generate 
// Random string of length 8
echo random_strings(8);

?>

Output

12945f0845
12945f08

Using sha1() Function: This function calculates the sha-1 hash of a string. Pass timestamps as a argument and sha1() function will convert them into sha1- hash.
Example:

PHP

<?php

  // This function will return 
  // A random string of specified length
  function random_strings($length_of_string) {
      
   
    // sha1 the timestamps and returns substring
    // of specified length
    return substr(sha1(time()), 0, $length_of_string);
}

// This function will generate 
// Random string of length 10
echo  random_strings(10);

echo "\n";

// This function will generate 
// Random string of length 8
echo  random_strings(8);

?>

Output

643f60c52d
643f60c5

Using random_bytes() Function: This function generates cryptographically secure pseudo-random bytes. It returns a string containing the requested number of cryptographically secure random bytes. Use bin2hex () function to convert bytes into hexadecimal format.
Example:

PHP

<?php

// This function will return
// A random string of specified length
function random_strings($length_of_string) {

    // random_bytes returns number of bytes
    // bin2hex converts them into hexadecimal format
    return substr(bin2hex(random_bytes($length_of_string)), 
                                      0, $length_of_string);
}

// This function will generate
// Random string of length 10
echo random_strings(10);

echo "\n";

// This function will generate
// Random string of length 8
echo random_strings(8);

?>

Output

64713970f3
67b575a3

Time complexity: O(length_of_string).

Space complexity: O(length_of_string).

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