preg_replace n次匹配的字符串？

Question

this should be very simple question, but i cant get it working for some reason, my framework is PHALCON

$string='GuGuSy';

i want this to be replaced as

$output = 'AAB';

so I coded it as:

 public function graha($planet)
    {
            if(strpos($planet,'Sy')!==false){
                $planet = preg_replace("/Sy/",'B',$planet);
            }
            if(strpos($planet,'Gu')!==false){
                $planet = preg_replace("/Gu/",'A',$planet);
                    //str_replace("Gu",'A',$planet);
            }
      return $planet;
  }

but output I get is not correct, why?

My output is for the string is "A" only why ?

Answer 1

$string='GuGuSy';

function graha($planet)
{
$a='';   
if(strpos($planet,'Sy')!==false){
$a= $planet = preg_replace("/Sy/",'B',$planet); // $planet=GuGuB
}

if(strpos($planet,'Gu')!==false){
$a= preg_replace("/Gu/",'A',$planet);

}
return $a;       
}

echo graha($string);