How can PHP developers effectively troubleshoot and debug issues related to undefined variables, failed file inclusions, and directory access errors?

Issue: PHP developers can effectively troubleshoot and debug issues related to undefined variables, failed file inclusions, and directory access errors by utilizing error reporting, checking for variable existence before usage, using file_exists() function to verify file inclusions, and setting appropriate file permissions for directory access. Code snippet:

// Enable error reporting to display notices and warnings
error_reporting(E_ALL);
ini_set('display_errors', 1);

// Check if a variable is defined before using it
if (isset($variable)) {
    // Code to use the variable
}

// Verify file inclusion using file_exists() function
$filename = 'file.php';
if (file_exists($filename)) {
    include $filename;
} else {
    echo 'File does not exist.';
}

// Set appropriate file permissions for directory access
chmod('/path/to/directory', 0755);