What causes routes to be non-functional if index.php is missing?

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If the index.php file is missing, then the server cannot find the default file to load when a web page is accessed. This default file is usually set in the server configuration, and if it is not found, the server will return a 404 error indicating that the page is not found. As a result, any routes that depend on the index.php file will not function properly. This is because the server cannot locate the file to execute the required code to serve the page.