Key length available via either EVP_CIPHER_key_length or EVP_CIPHER_CTX_key_length. Would need to decide if we're going to support variable length ciphers or not. Eg, a signature of:

openssl_get_key_length(string $cipher): int|false

or

openssl_get_key_length(string $cipher, string $encryption_mode): int|false

There is a new function in PHP 8.2 called openssl_cipher_key_length that can get the cipher key length for a given cipher algorithm12. This function returns an integer representing the length in bytes of the key required for the cipher algorithm, or false on failure1. For example, the following code will output:

PHP

<?php
$method = 'AES-128-CBC';
var_dump(openssl_cipher_key_length($method));
?>
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If you are using an older version of PHP, you may have to use a workaround to get the key length. One possible solution is to use the openssl_get_cipher_methods function to get an array of available cipher methods, and then use a regular expression to extract the key length from the method name (https://github.com)(https://www.rtasks.online/) For example, the following code will output:

PHP

<?php
$cipher = 'aes-256-cbc';
$ciphers = openssl_get_cipher_methods();
foreach ($ciphers as $c) {
    if (preg_match('/^'.$cipher.'-(\d+)-/i', $c, $matches)) {
        $key_len = $matches[1] / 8;
        break;
    }
}
echo "Key length for {$cipher}: {$key_len}\n";
?>