Data encrypted with openssl_public_encrypt is different every time?

Question

why is the content of $encrypted every time different?

// aquire public key from server
$server_public_key = openssl_pkey_get_public(file_get_contents("C:\publickey.pem"));

// rsa encrypt
openssl_public_encrypt("123", $encrypted, $server_public_key);

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also I have tried this one

$publicKey = "file://C:/publickey.pem";
$privateKey = "file://C:/privatekey.pem";
$plaintext = "String to encrypt";

openssl_public_encrypt($plaintext, $encrypted, $publicKey);
$transfer = base64_encode($encrypted);
openssl_private_decrypt($encrypted, $decrypted, $privateKey);

echo $transfer;  //encrypted string

and $transfer is everytime a different string:...

Z1xyMUquARxcGjqjjSHNAm41CnHI02GXxLyFivvta8YhDkhRJdD4i3kx+8GElljdiSY/NMF9UD3ritWMLGmscdq/QyIf+geYxJFePNd1dNWg+V6zbAKRLaEpsU+aB87jiM/GjytLEkI63dku02BS0ZBgz9UZw/FDNaynV5bTTDM=

mRgLPsPtMoV9la7zzuU+cLzS5xMDp7QUmH6Iv4Sv4/FNjt62zcv9ZMWkfG3uVhS8Z1UDtGl+met1CYjBTcfjHCR6hahbwOkTCICXtkRQcc371vURW04XhQzMNgIIbvN5BBdmIyYI6alrS2vKUq7b3T0h8sJf36zh5CynYzyDCFU=

G5FhMoJGiUwEBvEOeZpDDrEXdxbWX5iaJ6F+VdYJ3CURPRMftskZNlDhat8gA5V0G+3nXVQZptkHjxMkOqPlmwJHjgIqAiFppHLpEKohyT9qNwkAR00Y6PiWrNUJPiEIZqXHAb8TS0AA0Quhc0UAwcc+I8NGOD59k8BrZE6Z5Ew=

Answer 1

The PKCS#1 encryption algorithm uses some random seed to make the cipher-text different every time.

This protects the cipher-text against several attacks, like frequency analysis, ciphertext matching. For example, if you were using a public key to encrypt all your password without randomness. All the same password will yield the same cipher-text. Someone can figure out all the popular passwords by checking the frequency of the cipher-text.

For symmetric key encryption, IV (Initial Vector) serves a similar purpose.