First thing I notice is this line:
elseif ($naam == admin || $naam == marcel || $naam == webmaster || $naam == crimebay){
Those strings (admin, marcel, webmaster, crimebay) should be in quotes. E.g.
elseif ($naam == 'admin' || $naam == 'marcel' || $naam == 'webmaster' || $naam == 'crimebay'){
You could probably make this a little easier to maintain if you turned those into names into an array of "reserved" names:
$reserved = array('admin', 'marcel', 'webmaster', 'crimebay');
/**
* The original if statements here....
*/
elseif(in_array($naam, $reserved)) {
/* The name is reserved */
}
I would not limit yourself to this 1 row logic:
elseif(mysql_num_rows($result) == 1){
For testing purposes, try out:
elseif(mysql_num_rows($result) > 0){