How to display an error for invalid input with php/html ?

31 Dec, 2020
 PHP can be easily embedded in HTML files and HTML codes can also be written in a PHP file. The thing that differentiates PHP from a client-side language like HTML is, PHP codes are executed on the server whereas HTML codes are directly rendered on the browser. To display error for invalid input with HTML and PHP. 
Approach:
Display error on invalid error when
input textbox is left empty.
input is wrong.
PHP code: The following is the code for “form.php” which is used in the HTML code in the later part.
To show invalid input in PHP, set the name of the input textbox which is in HTML. All the fields are first checked for empty fields and then it is validated for correctness. If all fields are correct then it shows the success message. If the input given by the user is wrong then it will show a message for ” Invalid input!!”.
PHP
<?php$nameError = "";$emailError = "";$passwordError = "";$mobileError = "";$success = "";   function validate_input($input) {    $input = trim($input);    $input = stripslashes($input);    $input = htmlspecialchars($input);    return $input;}   if(isset($_POST['form_submit'])) {    $name = $_POST['name'];    $password = $_POST['password'];    $email = $_POST['user_email'];    $mobile = $_POST['mobile'];         if (empty($_POST["name"])) {        $nameError = "Name is required";    } else {        $name = validate_input($_POST["name"]);                  if($name == 'chetan') {            $success= "Thank you ". $name.", ";            echo $success;        }    }         if (empty($_POST["email"])) {        $emailError = "Email is required";    } else {        $email = validate_input($_POST["email"]);                  if($email == 'test@email.com') {            $success= $email." is correct";            echo $success;        }    }           if (empty($_POST["password"])) {        $passwordError = "Password is required";    } else {        $password = validate_input($_POST["password"]);              if($password == 'test@123') {            $success= $password." is correct";            echo $success;        }    }       if (empty($_POST["mobile"])) {        $mobileError = "Mobile is required";    } else {        $mobile = validate_input($_POST["mobile"]);                  if($mobile == '123456789') {            $success= $mobile." is correct";            echo $success;        }    }    if(empty($success))        echo "Invalid input!!!";}?>
HTML code: The following code uses the above PHP “form.php” code.
HTML
<!DOCTYPE html><html lang="en" dir="ltr">  <head>    <meta charset="utf-8">    <title>Form</title></head>  <body>    <form action="form.php" method="POST">        <input type="text" name="name"             placeholder="Enter name">          <input type="password" name="password"            placeholder="Password">          <input type="email" name="user_email"             placeholder="yourname@gamil.com">          <input type="tel" name="mobile"             placeholder="Mobile no">          <button type="submit" name="form_submit">            Submit        </button>    </form></body>  </html>
Output:
Incorrect input by userInvalid input!!!
Invalid input!!!
Correct input by user Thank you chetan, 123456789 is correct
Thank you chetan, 123456789 is correct
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