Why is it important to avoid outputting content before using the header() function in PHP?

It is important to avoid outputting content before using the header() function in PHP because headers must be sent before any actual output to the browser. If content is output before headers are set, PHP will throw an error like "headers already sent." To solve this issue, make sure to set headers before any output is sent to the browser. 

<?php
// Correct way to set headers before any output
header("Content-Type: text/html");

// Output content after setting headers
echo "Hello, World!";
?>

Keywords

header function content output HTTP headers buffer redirection

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