Get first img in post if any

add_filter( 'the_content' , 'mh_get_first_img' );

function mh_get_first_img( $content ) {

$mh_url_regex = "/<img\ .*\/\>/";

// Get all the matches and populate them in an array
preg_match_all( $mh_url_regex , $content, $mh_matches );

if ( $mh_matches[0][0] ) {
        // Zap everything after the first closing '>'
        $mh_first_img = preg_replace( '/>.*$/' , '>', $mh_matches[0][0]);
        // Add the img HTML to the top of the post as an example.
        $content = '<p><pre>' . esc_html( $mh_first_img ) . '</pre></p>' . $content;
}

return $content;
}