[php] How to fix "Headers already sent" error in PHP

Generally this error arise when we send header after echoing or printing. If this error arise on a specific page then make sure that page is not echoing anything before calling to start_session().

Example of Unpredictable Error:

 <?php //a white-space before <?php also send for output and arise error
session_start();
session_regenerate_id();

//your page content

One more example:

<?php
includes 'functions.php';
?> <!-- This new line will also arise error -->
<?php
session_start();
session_regenerate_id();

//your page content

Conclusion: Do not output any character before calling session_start() or header() functions not even a white-space or new-line