@rxc
Hi RXC, I purposefully left the condition in if statement.
If the bold last page is not selected in back-end settings of WordPress SEO plugin, I think your condition will not work.
So, the condition to work should be :
if( strpos($output, '<strong class="breadcrumb_last"') > 0 || strpos($output, '<span class="breadcrumb_last"') > 0 )