Using PHP phpseclib RSA encryption does not produce same string each time

Hello Experts,

I am attempting to use PHP phpseclib RSA encryption to encrypt a string but each time I pass the same string it produces a different output. I am hoping someone can point where I am going wrong

Here is the code:
I will be appending to the URL the encrypted string in form of "accountLoginType|username|password" in order to connect a user to a 3rd party site that has provided me with the publickey.

<?php
 
require_once('Crypt/RSA.php');

function EncryptQueryString($accountLoginType, $username, $password) {
	return RSAEncrypt($accountLoginType . "|" . $username . "|" . $password);
}
function RSAEncrypt($dataToEncrypt) {
	$publicKey = '<RSAKeyValue>
                      <Modulus>uno9DsYcaZ1yAqY20nIM+YjYjjFsGx0DYm7lBGxbmVLLZTYc9MaI0Br+
                      8ElcuZVVNRmGeVBlkcHT3JpMDf/fiWSho6o0pRhQZmnG4RZtCWnGjFTV+
                      QWBYcuTGoQFKOtsrGqG16XwL2hPxqYW/7nzBVgAGe6myG3hMou8P4DSpjk=</Modulus>
                      <Exponent>AQAB</Exponent>
                      </RSAKeyValue>';
       $xml = new DOMDocument();
       $xml->loadXML($publicKey);

       $modulus = new Math_BigInteger(base64_decode($xml->getElementsByTagName('Modulus')->item(0)->nodeValue), 256);
       $exponent = new Math_BigInteger(base64_decode($xml->getElementsByTagName('Exponent')->item(0)->nodeValue), 256);
       $key = array('modulus' => $modulus, 'publicExponent' => $exponent);

       $rsa = new Crypt_RSA();

       $rsa->loadkey($key, CRYPT_RSA_PUBLIC_FORMAT_RAW);
       $rsa->setEncryptionMode(CRYPT_RSA_ENCRYPTION_PKCS1);

       $plainbytes = mb_convert_encoding($dataToEncrypt,"UTF-16LE", "auto");
       $res = $rsa->encrypt($plainbytes);

       return  base64_encode ( $res );
}

echo EncryptQueryString('accountLoginType', 'username', 'password');
?>

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