Pagination in PHP for image gallery that shows all images in folder

I am trying to add a pagination to my image gallery that I've made entirely in PHP.

I used this post for it as it was the best fit I found.
However, browsing around I noticed that it's not working overall.

For example, I have a folder with 206 files and within 11 pages (18 per page) only shows 196 images.
I have another with 35 files and it doesn't go past the first page and so 18 images.

This is the code, where am I going wrong?
Please speak like I'm stupid because all of this I have done it's been out of my brain with no previous study or knowledge of PHP. Thanks

<div>
<?php
$folder = './././';
$filetype = '*.jpg*';    
$files = glob($folder.$filetype);
$total = count($files);    
$per_page = 18;    
$last_page = (int)($total / $per_page);    
if(isset($_GET["page"])  && ($_GET["page"] <=$last_page) && ($_GET["page"] > 0) ){
    $page = $_GET["page"];
    $offset = ($per_page + 1)*($page - 1);      
}else{
    $page=1;
    $offset=0;      
}    
$max = $offset + $per_page;    
if($max>$total){
    $max = $total;
}
    
for ($i = $offset; $i< $max; $i++) {
    $image = $files[$i];
    $info = pathinfo($image);
    $resolution = getimagesize($image);
        $file = $files[$i];
        $path_parts = pathinfo($file);
        $filename = $path_parts['filename'];
        echo '<div class="thumbs">';
        echo '<a target="_blank" href="'.$file.'"><img src="'.$image.'"></a><br>';
        echo '<span class="thumb_size">' . strval($resolution[0]) . 'x' . strval($resolution[1]) . '</span>';
        echo '</div>';
}   
show_pagination($page, $last_page); 
    
?>
</div>
<?php function show_pagination($current_page, $last_page){
    echo '<div class="pagination">';
    if( $current_page > 1 ){
        echo '<span class="button-14"><a href="?page='.($current_page-1).'"> &lt;&lt;Previous Page</a></span>&nbsp;';
    }
    if( $current_page < $last_page ){
        echo '<span class="button-14"><a href="?page='.($current_page+1).'"> Next Page&gt;&gt;</a></span>';  
    }
    echo '</div>';
}?>

Solution

The $last_page is off by 1. Consider what you're doing here:

$last_page = (int)($total / $per_page);

If there's any remainder from that calculation, casting to int will drop that remainder. So the last incomplete page of records is never available.

Instead of casting to an int, use the ceil() function:

$last_page = ceil($total / $per_page);

That way if there's any remainder, even just a single extra record, the $last_page will always round up to account for that remainder.