PHP how encode/decode text with key?

Decrypt:

//Encryption
$result = mcrypt_ecb (MCRYPT_3DES, 'test', $string, MCRYPT_ENCRYPT);
//Decryption
$decrypt_result = mcrypt_ecb (MCRYPT_3DES, 'test', $result, MCRYPT_DECRYPT);

You need to change mode in arguments and pass encrypted values.

NOTE: mcrypt_generic() has also been DEPRECATED as of PHP 7.1.0.

Read manual: http://www.php.net/manual/en/function.mcrypt-ecb.php.

Better to use mcrypt_generic().

$cc = 'my secret text';
$key = 'my secret key';
$iv = '12345678';

$cipher = mcrypt_module_open(MCRYPT_BLOWFISH,'','cbc','');

mcrypt_generic_init($cipher, $key, $iv);
$encrypted = mcrypt_generic($cipher,$cc);
mcrypt_generic_deinit($cipher);

mcrypt_generic_init($cipher, $key, $iv);
$decrypted = mdecrypt_generic($cipher,$encrypted);
mcrypt_generic_deinit($cipher);

echo "encrypted : ".$encrypted;
echo "<br>";
echo "decrypted : ".$decrypted;

See https://gist.github.com/joashp/a1ae9cb30fa533f4ad94

Simple PHP encrypt and decrypt using OpenSSL from Joashp who will work for food

/**
 * simple method to encrypt or decrypt a plain text string
 * initialization vector(IV) has to be the same when encrypting and decrypting
 * 
 * @param string $action: can be 'encrypt' or 'decrypt'
 * @param string $string: string to encrypt or decrypt
 *
 * @return string
 */
function encrypt_decrypt($action, $string) {
    $output = false;
    $encrypt_method = "AES-256-CBC";
    $secret_key = 'This is my secret key';
    $secret_iv = 'This is my secret iv';
    // hash
    $key = hash('sha256', $secret_key);

    // iv - encrypt method AES-256-CBC expects 16 bytes - else you will get a warning
    $iv = substr(hash('sha256', $secret_iv), 0, 16);
    if ( $action == 'encrypt' ) {
        $output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
        $output = base64_encode($output);
    } else if( $action == 'decrypt' ) {
        $output = openssl_decrypt(base64_decode($string), $encrypt_method, $key, 0, $iv);
    }
    return $output;
}
$plain_txt = "This is my plain text";
echo "Plain Text =" .$plain_txt. "\n";
$encrypted_txt = encrypt_decrypt('encrypt', $plain_txt);
echo "Encrypted Text = " .$encrypted_txt. "\n";
$decrypted_txt = encrypt_decrypt('decrypt', $encrypted_txt);
echo "Decrypted Text =" .$decrypted_txt. "\n";
if ( $plain_txt === $decrypted_txt ) echo "SUCCESS";
else echo "FAILED";
echo "\n";