Square root of number in php by Pratik Pradhan

<html>
<head>
    <title>
        Square Root
    </title>
    <link rel="stylesheet" href="char.css">
    <style>
   *{
    margin:0px;
    padding:0px;
    font-family: cambria;
}
.nav{
    background-image: linear-gradient(to bottom right, rgb(178, 115, 207), rgb(59, 185, 108));
    padding:10px;
    color:white;
    text-decoration:bold;
    font-size:larger;
}
.footer{
    background-color:white;
    padding:5px;
    color:black;
    font-size:small;
    position:absolute;
    top:95%;
}
.form{
    border-radius: 10px;
    width: 250px;
    position:absolute;
    top:25%;
    left:30%;
    width:40%;
    padding-bottom: 10%;
    padding-top: 10%;
    box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19);
    text-align: center;
    background-image: linear-gradient(to bottom right, rgb(178, 115, 207), rgb(59, 185, 108));
}
input{
    padding: 2%;
}
.button{
    background-color: rgb(218, 39, 113);
    border-radius: 10px;
    color:black;
    text-transform: uppercase;
    text-decoration: solid;
    border: none;
}
    </style>
</head>
<body>
    <div class="nav">
    Program 3. Develop PHP Script to calculate Square root of a number.
    </div>
    <div class="form">
        <form method="post">
            <input type="number" name="num" placeholder="Enter Number">
            <input type="submit" class="button" value="Square Root">
        </form>
   </div>
   <?php
        function root($num)
        {
        $a=$num;
        $b=1;
        while($a>$b)
        {
        $a=($a+$b)/2;
        $b=$num/$a;
        }
        return $a;
        }
        echo("<Center><h3>Square Root is: ".root($_POST['num'])."<br></h3></Center>");
        ?>
 <footer class="footer">
        <center>Copyrights Reserved with &copy;pratikpradhan</center>
    <footer>
</body>
</html>
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