<?php

function sayiDuzenle($sayi){
    $sayi = str_replace('.','',$sayi);
    $sayi = str_replace(',','.',$sayi);

    return $sayi;
}

$say_1 = sayiDuzenle('100.500,30');
$say_2 = 2;

$sonuc = floatval($say_1) * $say_2;

echo number_format($sonuc,2);
?>