<?php

function encrypt_pkcs7($str, $key)
{
    $key = md5($key, true);
    
    $block = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_ECB);
    $pad = $block - (strlen($str) % $block);
    $str .= str_repeat(chr($pad), $pad);

	$ciphertext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $str, MCRYPT_MODE_ECB);
    return base64_encode($ciphertext);
}

echo encrypt_pkcs7('test', '123456');