Wbblite Error

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function userlevel($userposts,$regdate) {
 global $tpl;

 $regdate = (time()-$regdate)/86400;
 $exp = ceil($userposts*$regdate);
 if($exp!=0) $level = pow(log10($exp),2);
 else $level = 0;
 $showlevel = floor($level) + 1;
 $nextexp = ceil(pow(10,pow($showlevel,0.5)));
 if($showlevel==1) $prevexp = 0;
 else $prevexp = ceil(pow(10,pow($showlevel-1,0.5)));