Note to self:
When generating a new private key using openssl_pkey_new(), and specifying the private_key_bits parameter as a variable.
Be sure to cast the variable to an integer!
e.g.
$keysize = "2048";
$privateKey = openssl_pkey_new(array(
'private_key_bits' => $keysize, // ---> This won't work
'private_key_bits' => (int)$keysize, // ---> Correct
'private_key_bits' => 2048, // ---> Also correct.
'private_key_type' => OPENSSL_KEYTYPE_RSA,
));