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<?php
if(isset($_POST['sub_form'])){
    if($_POST['wt']<= 0 ||$_POST['ht'] <= 0) die("Enter valid values.");
    $wt = $_POST['wt'];
    $ht = $_POST['ht'];
    $ht = $ht * $ht;
    $bmi =     round($wt/$ht,2);
    if($bmi < 20)die( 'You are underweight. Your BMI is '.$bmi);
    if($bmi >25) die ('You are overweight. Your BMI is '.$bmi);
    echo "You weight is optimum. Your BMI is ".$bmi;
}
?>