title: Use of underscore in PHP code
body: <p>Firstly, to be clear, I'm not talking about the use of underscores as for visual separation of words in function names, etc., for example from the code below, <code>woocommerce_wp_text_input</code>.</p>
  
  <p>In following a particular PHP WordPress WooCommerce related <a href="http://www.remicorson.com/woocommerce-custom-fields-for-variations/" rel="nofollow noreferrer">tutorial</a>, I noticed the coder was randomly inserting underscores and multiple consecutive underscores, for example: <code>_text_field</code> and <code>__( 'Enter the custom value here.', 'woocommerce' )</code>.  This is either a convention that is used in demonstration code snippets or it has some PHP meaning.</p>
  
  <pre><code>    // Text Field
  woocommerce_wp_text_input( 
      array( 
          'id'          =&gt; '_text_field[' . $variation-&gt;ID . ']', 
          'label'       =&gt; __( 'My Text Field', 'woocommerce' ), 
          'placeholder' =&gt; 'http://',
          'desc_tip'    =&gt; 'true',
          'description' =&gt; __( 'Enter the custom value here.', 'woocommerce' ),
          'value'       =&gt; get_post_meta( $variation-&gt;ID, '_text_field', true )
      )
  );
  </code></pre>
  
  <p>Is there a convention regarding the use of an arbitrary number of underscores in coding demonstrations?  At first I suspected he was using underscores to highlight that certain values were specific to his case, but I really don't know.</p>