PHP - How to ensure that if a POST is not inserted, "Notice: Undefined index"

You could use the isset command to test whether the value was reported .

In this way:

<?php 
if(isset($_GET["email"])){
    echo  ($_GET["email"]) ; 
}
?>

Note: You'll notice that a lot of people use the at ("@") to omit errors, but it can cause a big headache in a situation where you need to find the errors and they do not appear because they are omitted. It would look like this:

<?php 
echo  @($_GET["email"]); 
?>

Read more about whether or not to use the at sign ("@") at: Why they say use Is it a bad practice to suppress mistakes?

There are ways to avoid the appearance of this error message, but, I'm not sure the focus of your application that is developing, a good practice is the validation of data entry, which can be done in HTML JS or PHP , as you are performing a simple submission using the action attribute, I recommend that you already validate by HTML , like this:

<form method="post" action="" >
    <input type="email" name="email" required />
</form>

One more thing I noticed is that by your description you are sending data with POST method and you are using the $_GET[] supervariable, which is wrong, the right method is GET logo $_GET[] , or method POST logo $_POST[] .

Use the filter_input function, it receives a constant INPUT_GET or INPUT_POST :

<?php
echo filter_input(FILTER_POST, 'email');

This function also allows you to apply filters to the read variable, see more information on Types of filter (documentation page without translation)