How do I debug a WordPress plugin?

Question

I've recently inherited a WordPress plugin that has a few bugs in it. My problem is that I'm also new to WordPress and I don't know how to log debug messages so that I can figure out what's going on.

I really just need a way to create a popup or log to a console.

Answer 1

There's this excellent Q&A at WordPress Stack Exchange, lots of knowledgeable folks explaining their debugging techniques: How do you debug plugins?

In the Javascript arena you basically need <script>console.log('the value is' + variable);</script>. And use Google Chrome inspector and/or Firebug.

In PHP, it depends on where things are happening or where you want the output.

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Debugging in WordPress

Official documentation in the Codex.

Example wp-config.php for Debugging

// Enable WP_DEBUG mode define( 'WP_DEBUG', true );  // Enable Debug logging to the /wp-content/debug.log file define( 'WP_DEBUG_LOG', true );  // Disable display of errors and warnings  define( 'WP_DEBUG_DISPLAY', false ); @ini_set( 'display_errors', 0 );  // Use dev versions of core JS and CSS files (only needed if you are modifying these core files) define( 'SCRIPT_DEBUG', true ); 

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Printing information to a log file

The following uses an OSX/Unix/Linux system path, adjust for Windows.

/* Log to File  * Description: Log into system php error log, usefull for Ajax and stuff that FirePHP doesn't catch  */ function my_log_file( $msg, $name = '' ) {     // Print the name of the calling function if $name is left empty     $trace=debug_backtrace();     $name = ( '' == $name ) ? $trace[1]['function'] : $name;      $error_dir = '/Applications/MAMP/logs/php_error.log';     $msg = print_r( $msg, true );     $log = $name . "  |  " . $msg . "\n";     error_log( $log, 3, $error_dir ); } 

Then, in you code call the function my_log_file( $post, 'The post contents are:' );

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Print directly in the rendered Html

/* Echo variable  * Description: Uses <pre> and print_r to display a variable in formated fashion  */ function echo_log( $what ) {     echo '<pre>'.print_r( $what, true ).'</pre>'; } 

And wherever needed use it like: echo_log( $post );.

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FirePHP

This extension will log information directly in the browser console. Refer to the following Q&A at WordPress Answers: How to use WP-FirePHP extension?.