Hello,
You can put arround the flags a tag <a>
and put the atributte href with the name of your two sites like this:
<a href="index_eua.php" target="self_"><img src="image/flag1.png" /></a>
<a href="index_greek.php" target="self_"><img src="image/flag2.png" /></a>
In my website i did almost it in the footer with the flags of my social midias, but i used the target=”_blank” because i wanted to open a new tab,like this:
<a href="https://www.sitepor500.com.br" target="_blank"><img src="image/facebook.png" /></a>