Debugging Errors When Building Custom Wordpress Shortcodes

Troubleshooting Common Errors in Custom WordPress Shortcodes

When building custom WordPress shortcodes, you may encounter various errors that prevent your shortcodes from functioning properly. Identifying and troubleshooting these errors is key to creating robust and reusable shortcodes.

Common errors include incorrect shortcode attribute parsing, failing to properly escape output, issues with enqueuing scripts and styles, plugin and theme conflicts, and problems with staging environments. By leveraging debugging techniques like wp_debug and error logs, you can narrow down errors and pinpoint their origin.

Parsing Shortcode Attributes

One frequent source of errors is improperly handling shortcode attributes. WordPress passes shortcode attributes as an associative array to your shortcode callback function. Failing to properly access and validate attributes will often trigger notices or unexpected output.

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When registering your shortcode, define allowed attributes and default values with the ‘atts’ parameter. Use shortcode_atts() to parse attributes into an extraction variable within your callback. Extract specific attributes with isset() to avoid undefined index notices.

/** Register Shortcode */
function custom_shortcode( $atts ) {

  // Attributes
  $atts = shortcode_atts(
    array(
      'title' => 'Default Title',
      'items' => 3,
    ), 
    $atts 
  );

  // Extract Attributes
  $title = isset($atts['title']) ? $atts['title'] : null; 
  $items = isset($atts['items']) ? intval($atts['items']) : 3;
  
  // Rest of shortcode logic
  return $output;
}
add_shortcode( 'custom', 'custom_shortcode' );

Use validate_atts() to check an attribute value against a list of allowed values. This avoids outputting unexpected attribute values added through user error or malicious injections.

$allowed_colors = array('blue','green','red');

function shortcode_callback( $atts ) {

  $atts = shortcode_atts( array(
    'color' => 'blue',
  ), $atts );

  $atts['color'] = validate_atts( $atts['color'], $allowed_colors );
  
  // Rest of shortcode logic
  
  return $output;
  
}

Escaping Shortcode Output

Failing to properly escape output is another common source of errors and vulnerabilities within shortcodes. Escaping ensures special characters, HTML tags, and JavaScript do not break your page layout or execute unexpectedly.

WordPress provides the esc_html(), esc_attr(), and esc_url() functions to escape output based on context. Always escape user-provided data from shortcode attributes before outputting.

$title = isset($atts['title']) ? esc_html($atts['title']) : null;

$link = isset($atts['link']) ? esc_url($atts['link']) : null;

Use esc_textarea() to escape textarea content before outputting it. For unordered lists of items, use esc_html() around each list item.

$comments = esc_textarea($atts['comments']);

$items = '';
foreach( $atts['items'] as $item ) {
  $items .= ''. esc_html($item) .''; 
}
$items .= '';

Enqueuing Shortcode Assets

Shortcodes that integrate custom scripts, stylesheets, or other assets may fail to load properly without enqueuing them correctly. Use wp_enqueue_script() when your shortcode relies on external JavaScript files. Similarly, wp_enqueue_style() when your shortcode depends on custom CSS styles.

// Register and enqueue script
function custom_shortcode_scripts() {
  
  wp_register_script( 'custom-script', '/scripts/custom.js' );
  
  wp_enqueue_script('custom-script');

}
add_action( 'wp_enqueue_scripts', 'custom_shortcode_scripts' );

// Shortcode callback
function custom_shortcode() {

  // Shortcode logic
  
  return $output;

}

When enqueuing scripts and styles, ensure you use unique handle names that do not conflict with existing assets. Use dependencies and version numbers to control script load order and cache busting.

Debugging with wp_debug and var_dump()

Two invaluable tools for debugging errors in WordPress shortcodes are the wp_debug constant and PHP’s var_dump() function.

Enabling wp_debug in your site’s wp-config.php file will force WordPress to display notices, warnings and fatal errors instead of failing silently. Error messages triggered by your shortcode are then visible.

// Enable WP_DEBUG mode
define( 'WP_DEBUG', true );

Include calls to var_dump() within your shortcode callback to print formatted contents of a variable for inspection. Viewing attribute values and extracted data helps narrow down unexpected behavior.

function custom_shortcode( $atts ) {

  // Debug attributes
  var_dump($atts); // Outputs structured info 
  die(); // Ends execution
  
  // Rest of shortcode
  
  return $output;

}

Remove or comment out var_dump() lines after debugging to avoid printing raw data on the frontend.

Checking Error Logs

Reading your WordPress error log files provides additional context when tracking down shortcode bugs. Enable WP_DEBUG_LOG in wp-config.php to log errors and notices to an error.log file stored in /wp-content/.

define('WP_DEBUG_LOG', true);

Examine error logs after triggering your shortcode for relevant warning messages, file paths, and stack traces indicating where an error originates.

WordPress may struggle to write logs due to permissions issues. Ensure the /wp-content/ directory is writable by the server using CHMOD.

# Enable write permissions for web server user
chmod 755 /var/www/vhosts/example.com/httpdocs/wp-content

Inspect PHP and web server logs in addition to WordPress logs for possible runtime errors and variance in file locations.

Identifying Theme and Plugin Conflicts

Shortcode errors can stem from conflicts with active plugins and themes – identifying conflicts involves selectively disabling plugins and switching to default themes.

Test your shortcode while operating in WordPress Safe Mode, where all plugins are disabled. Re-enable plugins one-by-one to isolate the source of the conflict.

# Add to wp-config.php to enable safe mode
define('WP_SAFE_MODE', true);

Switching to a default theme like Twenty Nineteen eliminates custom theme code as the culprit. If your shortcode operates normally, inspect child theme files and comment out customizations until you pinpoint incompatible code.

Review recently updated plugins and themes as likely sources of fresh conflicts. Updating or rolling back changes may resolve unexpected shortcode errors that previously operated without issues.

Testing on a Staging Environment

Attempting to debug errors on a live, production website is risky. Instead, replicate issues by testing your shortcode on a staging or development environment.

A staging site is an identical clone of your production site running on a separate server. Debugging safely on staging prevents disruptions to visitors and avoids exposing debug output publicly.

Log in to staging admin to activate plugins, apply code edits, and trigger shortcodes using sample posts or front-end tests. Debug errors freely without worrying about site visibility or uptime.

Many managed WordPress hosts include integrated staging environments. You can also use plugins like WP Staging or Duplicator to generate a WordPress copy on localhost for development and debugging purposes.

Useful Debugging Plugins and Tools

Specialized tools are available alongside wp_debug and error logging to help examine shortcode execution flows and pinpoint faulty logic:

Debug Bar – Inspects PHP, SQL queries, request parameters, and post meta during execution.
Log Deprecated Notices – Logs usage of deprecated WordPress functions.
Query Monitor – Checks database impact and performance for requests.
XS Debug – Visual editor for debugging CSS and JavaScript.
Who Called the Function – Traces origins of function calls.
Debug Console – Browser console mirroring the native dev tools.

Combining these debugging solutions with error logging best practices will help identify and resolve common WordPress shortcode errors.

Debugging Errors When Building Custom WordPress Shortcodes