[WordPress] Post Internal custom Pagination

WP_Post_Pagination.php

<?php

// Custom Post Pagination
function custom_wp_link_pages( $args = '' ) {
	global $post;

	// Get Post Type name
	$post_type_name = ucfirst(get_post_type(get_the_ID()));

	$defaults = array(
		'before' => '<li><span class="pagination pagination_title">'.$post_type_name.' Sections:', 
		'after' => '</span></li>',
		'text_before' => '',
		'text_after' => '',
		'next_or_number' => 'number', 
		'nextpagelink' => 'Next page',
		'previouspagelink' => 'Previous page',
		'pagelink' => '%',
		'echo' => 1
	);

	$r = wp_parse_args( $args, $defaults );
	$r = apply_filters( 'wp_link_pages_args', $r );
	extract( $r, EXTR_SKIP );

	global $page, $numpages, $multipage, $more, $pagenow;

	$output = '';
	if ( $multipage ) {
		if ( 'number' == $next_or_number ) {
			$output .= $before;
			for ( $i = 1; $i < ( $numpages + 1 ); $i = $i + 1 ) {
				$j = str_replace( '%', $i, $pagelink );
				$output .= ' ';
				if ( $i != $page || ( ( ! $more ) && ( $page == 1 ) ) ) :
					// Add class to the link
					$add_class = str_replace('<a', '<a class="pagination" ', _wp_link_page( $i ));
					$output .= '<li>'.$add_class;
				else :
					$output .= '<li><span class="pagination current" title="This Page">';
				endif;

				$output .= $text_before . $j . $text_after;

				if ( $i != $page || ( ( ! $more ) && ( $page == 1 ) ) )
					$output .= '</a></li>';
				else
					$output .= '</span></li>';
			}
			$output .= $after;
		} else {
			if ( $more ) {
				$output .= $before;
				$i = $page - 1;
				if ( $i && $more ) {
					$output .= '<li>'. _wp_link_page( $i );
					$output .= $text_before . $previouspagelink . $text_after . '</a></li>';
				}
				$i = $page + 1;
				if ( $i <= $numpages && $more ) {
					$output .= '<li>'._wp_link_page( $i );
					$output .= $text_before . $nextpagelink . $text_after . '</a></li>';
				}
				$output .= $after;
			}
		}
	}

	if ( $echo )
		echo $output;

	return $output;
}

// Add Post Pagination Button -> Next Page in TinyMCE Editor
// https://shellcreeper.com/?p=889
function add_next_page_button( $buttons, $id ){
    /* only add this for content editor */
    if ( 'content' != $id )
        return $buttons; 
    /* add next page after more tag button */
    array_splice( $buttons, 13, 0, 'wp_page' );
       return $buttons;
}
add_filter( 'mce_buttons', 'add_next_page_button', 1, 2 ); // 1st row

?>