What is the issue with the "Undefined index: version" notice in the PHP code provided?

The issue with the "Undefined index: version" notice in the PHP code is that the code is trying to access an array key 'version' that does not exist in the $_POST array. To solve this issue, you should first check if the 'version' key exists in the $_POST array before trying to access it to avoid the notice. 

if(isset($_POST['version'])) {
    $version = $_POST['version'];
    // continue with the code that uses $version
} else {
    // handle the case when 'version' is not set in $_POST
}

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