wp_redirect() not working in Insert PHP plugin in WordPress

Question

This is my code in insert PHP plugin. I have added this PHP snippet generated by the plugin in a page in WordPress. The wp_redirect is not working with this snippet.

// define variables and set to empty values
$emailErr = $passwordErr = "";
$email = $password = "";

function test_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}



if ($_SERVER["REQUEST_METHOD"] == "POST") {
  if (empty($_POST["email"])) {
    $emailErr = "Email is required";
  } 
  else {
    $email = test_input($_POST["email"]);
    // check if e-mail address is well-formed
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
      $emailErr = "Invalid email format"; 
    }
  }

  if (empty($_POST["password"])) {
    $passwordErr = "Password is required";
  } else {
    $password = test_input($_POST["password"]);
    // check if URL address syntax is valid (this regular expression also allows dashes in the URL)
    //if (!preg_match("/\b(?:(?:https?|ftp):\/\/|www\.)[-a-z0-9+&@#\/%?=~_|!:,.;]*[-a-z0-9+&@#\/%=~_|]/i",$website)) {
    //  $websiteErr = "Invalid URL"; 
    //}


  if ($emailErr == "" && $passwordErr == "")
  {
  wp_redirect("https://grenstonhrs.000webhostapp.com",301);
  exit;
  }
  }

}

  ?>
  <div>
  <form id="form" method="POST">
Email
<br/>
<input name="email" style="width: 300px;" type="text" value="<?php echo $email;?>" />&nbsp;&nbsp;
<span style="color: #FF0000"><?php echo $emailErr;?></span>
<br/>
Password
<br/>
<input name="password" style="width: 300px;" type="password" />&nbsp;&nbsp;
<span style="color: #FF0000"><?php echo $passwordErr;?></span>
<br/>
<br/>
<input id="loginButton" name="loginButton" type="submit" />
</form>&nbsp;
<br/>
<a href="https://grenstonhrs.000webhostapp.com/sign-up/">Sign Up</a>
</div>

Answer 1

I have used WordPress' custom POST handler functionality before to solve a problem similar to the issue that you are having. Here is documentation from the codex on this: https://codex.wordpress.org/Plugin_API/Action_Reference/admin_post_(action)

Basically, you would use an action to handle the post request:

function my_handle_form_submit() {

    // This is where you will control your form after it is submitted, you have access to $_POST here.

}

// Use your hidden "action" field value when adding the actions
add_action( 'admin_post_nopriv_my_simple_form', 'my_handle_form_submit' );
add_action( 'admin_post_my_simple_form', 'my_handle_form_submit' );

And modify your form a but so that WordPress knows to use your function to handle the request:

<!-- Submit the form to admin-post.php -->
<form action="<?php echo esc_url( admin_url('admin-post.php') ); ?>" method="POST">

    <!-- Your form fields go here -->

    <!-- Add a hidden form field with the name "action" and a unique value that you can use to handle the form submission  -->
    <input type="hidden" name="action" value="my_simple_form">

</form>