luas segitiga

<?php
// Start the session
session_start();
?>
<!DOCTYPE html>
<html>
<head>
    <title>LUAS SEGITIGA</title>
</head>
<body>
<center>
<h2><u>LUAS SEGITIGA</u></h2>
<form action="" method="POST">
    <table>
        <tr><td>ALAS</td><td>:</td><td><input type="text" name="alas"></td></tr>
        <tr><td>TINGGI</td><td>:</td><td><input type="text" name="tinggi"></td></tr>
    </table>
    <input type="submit" name="proses" value="proses"><input type="reset" name="hapus" value="hapus">
</form>
<?php

    if(empty($_SESSION["alas"])){
        $alas=array();
        $tinggi=array();
        $proses=array();
    }
if (isset($_POST["proses"])) {
    if(isset($_SESSION['alas'])){
        $alas=$_SESSION['alas'];
        $tinggi=$_SESSION['tinggi'];
        $proses=$_SESSION['proses'];

    }

    array_push($alas,$_POST['alas']);
    array_push($tinggi,$_POST['tinggi']);
    $proses1 = $_POST['alas']*$_POST['tinggi']/2;
    array_push($proses,$proses1);

    $_SESSION["alas"]=$alas;
    $_SESSION["tinggi"]=$tinggi;
    $_SESSION["proses"]=$proses;

    echo "<script>alert(".count($alas).")</script>";

    if(count($alas)==5){
        $no = 1;
        echo "<center><table>";
        echo "<tr>
                <td>No</td>
                <td>Alas</td>
                <td>Tinggi</td>
                <td>Luas</td>
            <tr>";
        for($i=0; $i<5; $i++){
            echo "<tr>
                    <td>".$no."</td>
                    <td>".$alas[$i]."</td>
                    <td>".$tinggi[$i]."</td>
                    <td>".$proses[$i]."</td>
                <tr>";
            $no++;
        }
        echo "</table></center>";
        session_destroy();
    }

}
?>
</center>

</body>
</html>