PHP- Online Activity

<form action="rectangle.php" method="post">
<center>
<br>
<br>
<br>
<h2>Please enter the length and the width of your rectangle.</h2>
// I put placeholder
Enter length: <input type="text" name="textL" placeholder= "Enter number"/>
<br>
<br>
Enter width: <input type="text" name="textW" placeholder= "Enter number"/>
<br>
<br>
<input type="submit" value="Submit" />
</br>
<br>
</form>

<?php
if(isset($_POST['textL']) && isset($_POST['textW'])){
if(!empty($_POST['textL']) && !empty($_POST['textW'])){
if(!preg_match("([0-9])",$_POST['textL']) || !preg_match("([0-9])",$_POST['textW'])){
echo 'Enter only numbers.';
}
else{
$l = $_POST['textL'];
$w = $_POST['textW'];
echo "The rectangle with length $l and width $w has an area of ".calculate($l,$w);
}
}
}
// I use function to calculate the length and the width
function calculate($length, $width){
$area = $length * $width;
return $area;
}
?>