What are some best practices for troubleshooting PHP scripts that involve passing variables between different files?

When troubleshooting PHP scripts that involve passing variables between different files, a common issue is not properly including or requiring the necessary files where the variables are defined. To solve this, ensure that all files are properly included or required in the correct order. Additionally, check for any typos or errors in variable names or file paths.

// File 1: variables.php
$var1 = "Hello, ";
$var2 = "world!";

// File 2: index.php
include 'variables.php';
echo $var1 . $var2;