How can including external files impact the syntax and functionality of PHP code?

When including external files in PHP code, it is important to ensure that the included files do not contain syntax errors or conflicting function or variable names that could impact the functionality of the code. To avoid issues, it is recommended to use include_once or require_once statements to prevent multiple inclusions of the same file, which can lead to redeclaration errors. 

<?php
include_once 'external-file.php';
// rest of the PHP code
?>

Keywords

include require external files syntax error functionality

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