Hoe een custom field te tonen via PHP (wordpress)

Hier mijn vraag:

My woo-commerce products have a custom field. I would like to place the value of this custom field through PHP on my page. This is currently my code:
$content .= ‘<td>’ . get_the_title() . ‘</br>’ . <?php echo get_post_meta( get_the_ID(), ‘customfieldname’, true ); ?> . ‘</td>’;}

It is part of a php file from a plugin. This is the line without my (attempt) added:
$content .= ‘<td>’ . get_the_title() . ‘CUSTOMFIELD</td>’;}

The placement of my code is exactly where it has to be (checked this with the hardcoded string CUSTOMFIELD first.. that works and displays the word where it needs to be)… but I can’t figure out how to change my hardcoded string with my custom field