How can the order of code execution in PHP, such as the placement of the DOCTYPE declaration, affect the functionality of cookies and conditional display of content?

The order of code execution in PHP can affect the functionality of cookies and conditional display of content because certain headers, like the DOCTYPE declaration, need to be set before any output is sent to the browser. This can impact the ability to set cookies or conditionally display content based on certain criteria. To ensure proper functionality, headers should be set before any output is generated in the PHP script. 

<?php
// Set the DOCTYPE declaration before any output
header('Content-Type: text/html; charset=utf-8');
echo "<!DOCTYPE html>";
// Set cookies or conditional content display after setting headers
setcookie("user", "John Doe", time() + 3600, "/");
if(isset($_COOKIE['user'])){
    echo "Welcome back, ".$_COOKIE['user']."!";
} else {
    echo "Welcome, guest!";
}
?>

Keywords

DOCTYPE declaration code execution order cookies conditional display PHP

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