auto-chosen feature image

if a user didnt select feature image, site will automatically choose the first image of article as a feature image.
i used this code.

//function to call first uploaded image in functions file
function main_image() {
$files = get_children('post_parent='.get_the_ID().'&post_type=attachment
&post_mime_type=image&order=desc');
  if($files) :
    $keys = array_reverse(array_keys($files));
    $j=0;
    $num = $keys[$j];
    $image=wp_get_attachment_image($num, 'large', true);
    $imagepieces = explode('"', $image);
    $imagepath = $imagepieces[1];
    $main=wp_get_attachment_url($num);
        $template=get_template_directory();
        $the_title=get_the_title();
    print "<img src='$main' alt='$the_title' class='frame' />";
  endif;
}

now i need to replace the fuction post_thumbnail() with the following

<?php if (  (function_exists('has_post_thumbnail')) && (has_post_thumbnail())  ) {
  echo get_the_post_thumbnail($post->ID);
} else {
   echo main_image();
} ?>

but i cant find the post_thumbnail() function
is there any other way to do that?
note: i dont want to use plugin.