What is the difference between include and require in PHP?

Both include and require insert the contents of another PHP file into the current script at the point they are called. The key difference is error handling when the file is not found: include produces a warning (E_WARNING) and continues executing the script. require produces a fatal error (E_COMPILE_ERROR) and halts execution. Use require for files that are essential to the application (configuration, database connections), and include for optional parts. Their _once variants (include_once, require_once) ensure the file is included only once, preventing function redeclaration errors.