PHP Function Call within a PHP

Hello,
I was trying my luck at coding a small WordPress plugin.. But, I am stuck at a point.

How can I do this?

<?php

function myfunc($content) {

$content.= '<span style="margin-top:10px" class="avatar"><?php echo get_avatar(get_the_author_email(), 70);?></span>';

return $content;
}
add_filter ('the_content', 'end_post');

?>

PHP:

When ever I put that, the <?php and ?> at the $content. automatically turns into <!--php and -->.
How do I call that gravatar (or any function) there at the $content?

 

There is no need to parse php in php. The function can just be executed the fist time:

function myfunc($content) {

$content.= '<span style="margin-top:10px" class="avatar">'. get_avatar(get_the_author_email(), 70).'</span>';

return $content;
}

PHP:

 

Awesome!!!
Thanks