Add dynamic sku to img src-tag (Magento 2)

I need to change the img src dynamic after what product it is on for it to show the right image. Here is the code i have tried. but the php-part doesn't work (it doesn't echo sku, the <php> tag shows on the product page). <div class="custom-block"> <a aria-hidden="true" href="[login to view URL]<?php /* @escapeNotVerified */ echo $_product->getSku() ?>" rel="nofollow"> <img style="width: 150px; height: auto;" src="[login to view URL]<?php /* @escapeNotVerified */ echo $_product->getSku() ?>" /> </a></div> And a link to a product page: [login to view URL] (the image is right below the add to cart-button)