function mobileReplace($mobile,$start,$end,$str="*"){
    $countStr = abs($end-$start);
    $replaceStr = $str;
    for($i=0;$i<$countStr;$i++){
        $replaceStr.= $str;
    }
    $mobile = substr_replace($mobile, $replaceStr, $start-1, $countStr+1);
    return $mobile;;
}

$mobile = '13188888888'; 
echo mobileReplace($mobile,4,7); //132****8888