Convert public part of OpenSSL RSA private key to SSH authorized_keys format

get-key.php

<?php

function encodeMPINT($binary)
{
    // If the most significant bit is set, prepend a zero byte to make it positive
    if (ord($binary[0]) & 0x80) {
        $binary = "\x00".$binary;
    }

    // Encode the length as a 32-bit big-endian integer
    $length = strlen($binary);
    $lengthBytes = pack('N', $length);

    // Combine the length and binary data
    return $lengthBytes.$binary;
}

// Load private key from file key.txt
$pkey = openssl_pkey_get_private(file_get_contents('key.txt'));
// Get details of the key
$details = openssl_pkey_get_details($pkey);

echo 'ssh-rsa '.base64_encode(encodeMPINT('ssh-rsa').encodeMPINT($details['rsa']['e']).encodeMPINT($details['rsa']['n']));