Set the uploaded image as Featured image of post - php

Question

i try to make a form where users can upload a resource with a thumbnail, 3 preview images and a .rar, .zip file.
The form works and creates me a post in my custom post type.
The image so far gets uploaded too in my uploads folder.
but im not able to set the uploaded image as the featured image of the new created post.
i found alot of solutions that should work, but i cant get it to work.
Maybe someone can tell me how to do it properly :)
Below is my code that creates me the new post and saves the image in my uploads folder.
<?php
if( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_POST['action'] ) && $_POST['action'] == "new_resource") {
// Do some minor form validation to make sure there is content
if (isset ($_POST['title'])) {
$title = $_POST['title'];
} else {
echo 'Please enter a title';
}
if (isset ($_POST['description'])) {
$description = $_POST['description'];
} else {
echo 'Please enter the content';
}
$tags = $_POST['post_tags'];
// Add the content of the form to $post as an array
$new_post = array(
'post_title' => $title,
'post_content' => $description,
'post_category' => array($_POST['cat']), // Usable for custom taxonomies too
'tags_input' => array($tags),
'post_status' => 'publish', // Choose: publish, preview, future, draft, etc.
'post_type' => 'community-resources' //'post',page' or use a custom post type if you want to
);
//save the new post
$pid = wp_insert_post($new_post);
wp_redirect(get_permalink($pid)); exit;
//insert taxonomies
$target_dir = "uploads/community/";
$target_file = $target_dir . basename($_FILES["thumbnail"]["name"]);
$uploadOk = 1;
$imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));
// Check if image file is a actual image or fake image
if(isset($_POST["submit"])) {
$check = getimagesize($_FILES["thumbnail"]["tmp_name"]);
if($check !== false) {
echo "File is an image - " . $check["mime"] . ".";
$uploadOk = 1;
} else {
echo "File is not an image.";
$uploadOk = 0;
}
}
// Check if file already exists
if (file_exists($target_file)) {
echo "Sorry, file already exists.";
$uploadOk = 0;
}
// Check file size
if ($_FILES["thumbnail"]["size"] > 10000000) {
echo "Sorry, your file is too large.";
$uploadOk = 0;
}
// Allow certain file formats
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
&& $imageFileType != "gif" ) {
echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
$uploadOk = 0;
}
// Check if $uploadOk is set to 0 by an error
if ($uploadOk == 0) {
echo "Sorry, your file was not uploaded.";
// if everything is ok, try to upload file
} else {
if (move_uploaded_file($_FILES["thumbnail"]["tmp_name"], $target_file)) {
echo "The file ". htmlspecialchars( basename( $_FILES["thumbnail"]["name"])). " has been uploaded.";
} else {
echo "Sorry, there was an error uploading your file.";
}
}
add_post_meta($pid, '_thumbnail_id', $attach_id);
}
} ?>
and here is my form:
<div id="upload_resource_wrapper" class="container">
<form method="post" enctype="multipart/form-data" id="new_resource" name="new_resource">
<label for="title">Title:
<input id="title" tabindex="1" name="title" size="20" type="text" value="" placeholder="Name of resource">
</label>
<label for="description">Description:<br>
<textarea id="description" tabindex="3" cols="50" name="description" rows="6" placeholder="Some text about the resource"></textarea>
</label>
<label for="Category">Category:<br>
<?php wp_dropdown_categories( 'tab_index=3&taxonomy=category' ); ?>
</label>
<label for="model_file">Upload the file here:<br>
<input accept="zip,application/octet-stream,application/zip,application/x-zip,application/x-zip-compressed" name="model_file" size="50" type="file">
</label>
<label for="thumbnail">Upload the main thumbnail<br>
<input accept=".png, .jpg, .gif, .jpeg. ,webp" name="thumbnail" size="50" type="file">
</label>
<label for="first_preview">Upload the first preview<br>
<input accept=".png, .jpg, .gif, .jpeg. ,webp" name="first_preview" size="50" type="file" />
</label>
<label for="second_preview">Upload the second preview<br>
<input accept=".png, .jpg, .gif, .jpeg. ,webp" name="second_preview" size="50" type="file" />
</label>
<label for="third_preview">Upload the third preview<br>
<input accept=".png, .jpg, .gif, .jpeg. ,webp" name="third_preview" size="50" type="file" />
</label>
<label for="post_tags">Tags<br>
<input id="post_tags" tabindex="5" name="post_tags" size="16" type="text" value="">
</label>
<p>
<input action="new_resource" id="submit" tabindex="6" name="submit" type="submit" value="Publish" />
</p>
<input name="action" type="hidden" value="new_resource" />
<?php wp_nonce_field( 'new-post' ); ?>
</form>
</div>
I tryd to use
"add_post_meta($post_id, '_thumbnail_id', $attach_id);"
but dont know how to get the thumbnail id.
i also tried:
"wp_insert_attachment( $attachment(dont get this),
$_FILES["thumbnail"]["tmp_name"], 209 );
i got nothing to work :(