add_filter( 'the_content', 'filter_function_name_11' );
function filter_function_name_11( $content ) {
$extra_text = 'blabla';
$paragraphs = explode('</p>',$content);
$new_content = '';
foreach ($paragraphs as $index => $paragraph) {
$new_content .= $paragraph;
if( round( count($paragraphs) / 2 ) == $index ) {
$new_content .= $extra_text;
}
}
return $new_content;
}