title: php apply filters is not working
body: <p>I got a problem with my php code. I use woocommerce and have to apply a filter to the products on the cart-page with some information of the product. I've used apply_filters but can't get it to work.</p>
  
  <p>Here is code from the woocommerce-file that works fine with item_weight but not area. </p>
  
  <pre><code>&lt;p&gt;Stk/m&lt;sup&gt;2&lt;/sup&gt;:&lt;/p&gt;
  &lt;?php echo apply_filters( 'woocommerce_cart_item_weight', $_product-&gt;get_weight()); ?&gt;
  &lt;p&gt;Test&lt;sup&gt;2&lt;/sup&gt;:&lt;/p&gt;
  &lt;?php echo apply_filters( 'woocommerce_area_unit' $thepostid-&gt;get_area());?&gt;
  &lt;span&gt;&lt;?php echo($_area)?&gt;M2&lt;span&gt;
  </code></pre>
  
  <p>And here is code from the other plugin that should work with the code above.</p>
  
  <pre><code>function wc_price_calculator_product_options_dimensions() {
  global $thepostid;
  
  woocommerce_wp_text_input( array( 'id' =&gt; '_area', 'label' =&gt; __( 'Area', WC_Measurement_Price_Calculator::TEXT_DOMAIN ) . ' (' . get_option( 'woocommerce_area_unit' ) . ')', 'description' =&gt; __( 'Overrides the area calculated from the width/length dimensions for the Measurements Price Calculator.', WC_Measurement_Price_Calculator::TEXT_DOMAIN ) ) );
  </code></pre>
  
  <p>Hope you can help me out. Thanks in advance.</p>