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.