How can PHP developers prevent header modification errors when working with custom functions in WordPress themes?

When working with custom functions in WordPress themes, PHP developers can prevent header modification errors by ensuring that headers are not already sent before attempting to modify them. This can be achieved by using output buffering functions like ob_start() and ob_end_flush() to capture and manipulate output before it is sent to the browser.

// Start output buffering
ob_start();

// Your custom functions and code here

// End output buffering and flush output
ob_end_flush();