how to validate input data in php

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validation

Hi.
I am new. I don't know how to validate certain data like emai, web site and numeric value. Here is my code which is work fine. Below is my code.

Pls Help me

add.php

<?php

include_once '../inc/header.php';
?>
<Script Language="javascript">
function change_action()
    {
        var frm_obj=document.getElementById("frm");
        frm_obj.action="data.php";
    }
</Script>


<form action="" method="POST" id="frm" >
    <table class=""  align="center" cellpadding="4" cellspacing="1">  
    <tr>
        <td><label class="lbl">ID </label></td>
        <td><label class="lbl">: </label></td>
        <td><input type="text" name="id" id="id" readonly="" class="txt"></td>
    </tr>    
    <tr>
        <td><label class="lbl">Name </label></td>
        <td><label class="lbl">: </label></td>
        <td><input type="text" name="name" id="name" class="txt"></td>
    </tr>
    <tr>
        <td><label class="lbl">Address </label></td>
        <td><label class="lbl">: </label></td>
        <td><textarea  name="address" id="address" class="txt" rows="5" cols="40"></textarea></td>
    </tr>
   <tr>
        <td><label class="lbl">Tel</label></td>
        <td><label class="lbl">: </label></td>
        <td><input type="text" name="tel" id="tel"  class="txt"></td>
    </tr>  
   <tr>
        <td><label class="lbl">Fax</label></td>
        <td><label class="lbl">: </label></td>
        <td><input type="text" name="fax" id="fax" class="txt"></td>
    </tr>      
   <tr>
        <td><label class="lbl">Email</label></td>
        <td><label class="lbl">: </label></td>
        <td><input type="text" name="email" id="email" class="txt"></td>
    </tr>    
   <tr>
        <td><label class="lbl">Web site</label></td>
        <td><label class="lbl">: </label></td>
        <td><input type="text" name="website" id="website" class="txt"></td>
    </tr>  
   <tr>
        <td><label class="lbl">Type</label></td>
        <td><label class="lbl">: </label></td>
        <!--<td><input type="text" name="type" id="type" class="txt"></td>-->
        <td>
            <SELECT NAME=type id="type">
                <OPTION VALUE=0>Choose
                <?php  
                    $sql="SELECT * FROM bk_parameter where type='typcs'  order by id";
                    $result = mysqli_query($con,$sql);
                    while($row = mysqli_fetch_array($result))
                       {
                            $typename=$row["name"];
                            echo "<OPTION VALUE=\"$typename\">".$typename.'</option>';
                       }
               ?>
             </SELECT>
       </td>
   </tr> 
   <tr>
      
       <td></td>
       <td></td>
       <td><input type="submit" value="submit" name="submit" class="btn" onclick="change_action()">
           <input type="submit" value="back" name="back" class="btn" onclick="change_action()"></td>
   </tr>
         
</table>
</form>
   
<?php
include_once '../inc/footer.php';
?>

data.php

<?php
include_once '../inc/header.php';

//insert into table

if (isset($_POST['submit']) && $_POST['submit']  != "" )
    {

        $name = ($_POST["name"]);
        $address = ($_POST["address"]);
        $tel = ($_POST["tel"]);
        $fax = ($_POST["fax"]);
        $email = ($_POST["email"]);
        $website = ($_POST["website"]);
        $type = ($_POST["type"]);    
       try 
            { 
                $sql="INSERT INTO bk_customer (name,address,tel,fax,email,website,type)
                      VALUES ('$name','$address','$tel','$fax','$email','$website','$type')";
                $result = mysqli_query($con,$sql) ; 
                
            header("Location:index.php"); 
            exit();
            } 
        catch (Exception $ex) 
            {
               echo $e->getMessage() . "\n";
               file_put_contents('PDOErrors.txt', $e->getMessage(), FILE_APPEND);
               exit();            
            }
     }
//mysqli_close($con);

// update table
if (isset($_POST['update']) && $_POST['update']  != "" )
    
    {
        $id= ($_POST["id"]);
        $name = ($_POST["name"]);
        $address = ($_POST["address"]);
        $tel = ($_POST["tel"]);
        $fax = ($_POST["fax"]);
        $email = ($_POST["email"]);
        $website = ($_POST["website"]);
        $type = ($_POST["type"]);    
       
        try 
            {
            $sql="UPDATE bk_customer SET name='$name',address='$address', tel='$tel',fax='$fax',"
                    . "email='$email', website='$website',type='$type' WHERE id='$id'";
            $result = mysqli_query($con,$sql) ; 
            header("Location:index.php"); 
            } 
        catch (Exception $ex) 
            {
              echo $e->getMessage() . "\n";
              file_put_contents('PDOErrors.txt', $e->getMessage(), FILE_APPEND);
              exit();                   
            }
    }
// update record

if (isset($_POST['delete']) && $_POST['delete']  != "" )
    {
        $id=$_POST["id"];
        try
            {
            $sql="DELETE FROM bk_customer WHERE id='$id'";
            $result = mysqli_query($con,$sql) ; 
            header("Location:index.php");             
            } 
        catch (Exception $ex) 
            {
              echo $e->getMessage() . "\n";
              file_put_contents('PDOErrors.txt', $e->getMessage(), FILE_APPEND);
              exit();       
            }

    }

// back to index.php
    
if (isset($_POST['back']) && $_POST['back']  != "" )
    {
        header("Location:index.php"); 
    }