<?php

echo "<form action=\"\" method=\"post\">
<div align=\"center\">
<table border=\"0\" width=\"16%\" cellspacing=\"0\" cellpadding=\"0\">
    <tr>
        <td><input type=\"text\" name=\"kenar1\"></td>
    </tr>
    <tr>
        <td height=\"22\"><input type=\"text\" name=\"kenar2\"></td>
    </tr>
    <tr>
        <td><input type=\"text\" name=\"kenar3\"></td>
    </tr>
    <tr>
        <td><input type=\"submit\" value=\"Çevreyi Bul\"></td>
    </tr>
</table>
</div>
</form>";
if($_POST){
$kenar1 = $_POST["kenar1"];
$kenar2 = $_POST["kenar2"];
$kenar3 = $_POST["kenar3"];
if(empty($kenar1) or empty($kenar2) or empty($kenar3)){
    echo "<center><font color=red><h2>Üç Kenarı Yazmadan Çevremi Bulunur?</h2></font></center>";
}else{
    $cevrehesapla = $kenar1+$kenar2+$kenar3;
    echo "<center><h1>Çevre : $cevrehesapla</h1></center>";
}
}

?>