function hidtel($phone){
 $IsWhat = preg_match('/(0[0-9]{2,3}[\-]?[2-9][0-9]{6,7}[\-]?[0-9]?)/i',$phone); //固定電話
 if($IsWhat == 1){
 return preg_replace('/(0[0-9]{2,3}[\-]?[2-9])[0-9]{3,4}([0-9]{3}[\-]?[0-9]?)/i','$1****$2',$phone);
 }else{
 return preg_replace('/(1[358]{1}[0-9])[0-9]{4}([0-9]{4})/i','$1****$2',$phone);
 }
}

$num = "13966778888"
$str = substr_replace($num,'****',3,4);

$phonenum = "13966778888";
echo hidtel($phonenum);