Can someone please explain to me what this piece of php code means.
echo "<a href='product.php?product_id".$product_id."'>
Is it saying that the link is taken from a variable from the product.php page and its named $product_id?
This is a GET parameter in the URL (it's also wrong btw). Say $product_id = 1.
echo "<a href='product.php?product_id".$product_id."'>
This would be "product.php?product_id1"
echo "<a href='product.php?product_id=".$product_id."'>
This would be "product.php?product_id=1", which would would handle by using
$_GET["product_id"]; //yields 1
Ths snippet outputs an HTML <a> tag. The $product_id is a variable, and echo is the command to output a string. But I'd recommend to follow some basic Tutorials about HTML and PHP, as these are some of the most basic things.