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Form Validation is a necessary process before the data entered in the form is submitted to the database.

<!DOCTYPE HTML>  <html><head><style>.error {color: #FF0000;}</style></head><body><?php// define variables and set to empty values$nameErr = $emailError = $mobileError ="";$name = $email = $mobile =  "";if ($_SERVER["REQUEST_METHOD"] == "POST") {  if (empty($_POST["name"])) {    $nameErr = "Name is required";  }   else {    $name = test_input($_POST["name"]);    // check if name only contains letters and whitespace    if (!preg_match("/^[a-zA-Z ]*$/",$name)) {      $nameErr = "Only letters and white space allowed";     }  }  if (empty($_POST["name"])) {    $mobileError = "Name is required";  } else {    $mobile = test_input($_POST["mobile"]);    // check if name only contains letters and whitespace    if (!preg_match('/^[0-9]{10}+$/', $mobile)) {      $mobileError = "10 digit Number";     }  }  if (empty($_POST["email"])) {    $emailError = "Email is required";  } else {    $email = test_input($_POST["email"]);    // check if e-mail address is well-formed    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {      $emailError = "Invalid email format";     }  }}function test_input($data) {  $data = trim($data);  $data = stripslashes($data);  $data = htmlspecialchars($data);  return $data;}?><form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">    Name: <input type="text" name="name" value="<?php echo $name;?>">  <span class="error">* <?php echo $nameErr;?></span>  <br><br>  Mobile: <input type="text" name="mobile" value="<?php echo $mobile;?>">  <span class="error">* <?php echo $mobileError;?></span>  <br><br>  E-mail: <input type="text" name="email" value="<?php echo $email;?>">  <span class="error">* <?php echo $emailError;?></span>  <br><br>  <input type="submit" name="submit" value="Submit">  </form></body></html>

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