Redefining base class functions?

Is there a way (PHP-GTK1) to redefine a base class method? Specifically, I 
need to change set_usize() so that the values for X and Y can be 
recalculated. The function I'm envisioning might look like: 

define('SIZEMULTIPLIER', 1.5); 
... snip ...
function set_usize($x, $y) 
{ 
return 
 parent::set_usize(round($x * SIZEMULTIPLIER), round($y * SIZEMULTIPLIER)); 
} 

So far, I'm drawing blanks. Is this possible? Any ideas? 

Thanks, 

Ben 
--
Only those who reach toward a goal are likely to achieve it. 

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