if statement - PHP Parse error: syntax error, unexpected T_IF

Try rewriting your code
as:

while ($rij1 =
            mysql_fetch_object($result1))
{
 if ($voornaam === NULL)

            {
 $rij2 = ' ';
 }
 else
 {
 $rij2 =
            mysql_fetch_object($result2);

            }
}

Edit: Corrected
your condition in the first if, as @andrewsi spotted -
= is an assignment operator, so previously your code was
changing $voornaam to NULL, then
checking if the result evaluated to true (which, of course, it
never would - so the second block would always execute)

In
your original code, you're using the and operator - presumably
having seen it used in some well meaning but poorly coded examples like
mysql_connect(...) or die('an error
occurred');.

What's happening in that example
is that the result of the first statement - mysql_connect() -
is checked. If it evaluates to true, the second statement never executes, but if it
evaluates to false then the second statement - die('an error
occurred'); - is executed. As you've just discovered, this pattern is
confusing and best avoided.