How to decrypt "AES-256-ECB" Cypher text in php

I am trying to encrypt and then decrypt "AES-256-ECB" , This is how I am encrypting the plain text :

 $string     = "This is a test string";
 $cipher     = 'AES-256-ECB';
 $key        = 'FFVgT4ThOPzglvcaFDc21mg5hZsY73Tc';
 $plaintext  = $string;
 if (strlen($plaintext) % 16) 
 {
     $plaintext = str_pad($plaintext, strlen($plaintext) + 16 - strlen($plaintext) % 16, "\0");
 }
 $chiperRaw     = openssl_encrypt($plaintext, $cipher, $key, OPENSSL_NO_PADDING);
 $ciphertext = trim(base64_encode($chiperRaw)); 

But How can I decrypt the cyphertext back to plain text ?