How to Generate Secure Random Alphanumeric Strings for Verification Links in PHP?

Generating a Random, Unique Alphanumeric String for Verification Links in PHP

When registering on a website, you often receive an email containing a link to verify your account. This link requires a unique, random string to ensure its authenticity. Here's how to generate such a string in PHP.

Using PHP 7 Standard Library (Recommended)

PHP 7 introduced random_bytes($length) function for generating cryptographically secure pseudo-random bytes.

$bytes = random_bytes(20);
var_dump(bin2hex($bytes));

PHP 5 (Outdated)

For outdated PHP versions, use bin2hex(openssl_random_pseudo_bytes($bytes)), which generates a random alphanumeric string of length = $bytes * 2.

Enhanced Cryptographically Secure Function

The following function satisfies all security criteria:

function getToken($length)
{
    $token = "";
    $codeAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    $max = strlen($codeAlphabet);

    for ($i = 0; $i < $length; $i++) {
        $token .= $codeAlphabet[crypto_rand_secure(0, $max - 1)];
    }

    return $token;
}

crypto_rand_secure($min, $max) uses openssl_random_pseudo_bytes to generate a random number between $min and $max. getToken($length) creates a custom alphabet and generates a string of length $length.

Conclusion

Using these functions, you can generate robust random and unique strings for various security-related purposes, such as verification links and password retrieval tokens.

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