How to use a function inside of a function, or rewrite this code to work:

To ensure that the checkposts function works properly within a WordPress hook action, you can define the checkposts function outside the populate_inventory function. This approach keeps the code organized and ensures that the function is available when needed. Here’s the revised version of your code:

add_action( 'populate_inventory_hook', 'populate_inventory' );

function populate_inventory() {
    $url="https://website.com/webfeed?version=2";

    //  Initiate curl
    $ch = curl_init();
    // Will return the response, if false it print the response
    curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );
    // Set the url
    curl_setopt( $ch, CURLOPT_URL, $url );
    // Execute
    $result = curl_exec( $ch );
    // Closing
    curl_close( $ch );

    $items = json_decode( $result, true );
    $feed_count = count( $items );

    $check_counter = 0;
    $exist_counter = 0;
    $non_exist_counter = 0;

    $wp_args = array(
        'post_type'      => 'wheelchair-vans-sale',
        'posts_per_page' => -1,
        'post_status'    => 'publish',
    );

    $query = new WP_Query( $wp_args );
    if ( $query->have_posts() ) : 
        while ( $query->have_posts() ) : 
            $query->the_post(); 

            $check_counter++;

            if ( checkposts( $items, $post->ID ) ) {
                echo "1";
                $exist_counter++;     
            } else {
                echo "0";
                $non_exist_counter++;
            }

        endwhile; 
        echo "<br />";
        wp_reset_postdata();
    endif; 
}

function checkposts( $items, $postid ) {
    global $post;   
    $wp_vin = get_post_meta( $postid, "item_vin", true );        
    $i = 0;
    foreach ( $items as $item ) {   
        if ( $item['vin'] ==  $wp_vin ) {
            $i++;
        }       
    }     
    if ( $i < 1 ) {
        wp_delete_post( $postid );         
    }     
    return $i; 
}

In this setup, the checkposts function is defined separately and is called within the populate_inventory function to perform the necessary checks. This structure ensures that all functions are properly defined and accessible when the hook is executed.