How can the issue of undefined variables be resolved when including multiple files in PHP?

When including multiple files in PHP, the issue of undefined variables can be resolved by using the `require_once` or `include_once` functions instead of `require` or `include`. This ensures that the included file is only loaded once, preventing redeclaration of variables and avoiding the issue of undefined variables. 

// File: main.php

require_once 'file1.php';
require_once 'file2.php';

// Rest of the code

Keywords

undefined variables including files PHP error handling scope

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