How do I share variables between two functions?

Question

So...here is part of my code for a plugin. I'm a newb of sorts so be kind. I've read up on global variables but I can't seem to get it to work and I've read you shouldn't use them anyway. So what would be the best way to write the code below without having to re-declare the variables for each function? Here is the full code if necessary.

// Display the product badge on the shop page

add_action( 'woocommerce_after_shop_loop_item_title', 'wc_simple_product_badge_display_shop', 30 );
function wc_simple_product_badge_display_shop() {
    $title = get_post_meta( get_the_ID(), '_wc_simple_product_badge_title', true ); // badge title
    $class = get_post_meta( get_the_ID(), '_wc_simple_product_badge_class', true ); // badge class
    $duration = get_post_meta( get_the_ID(), '_wc_simple_product_badge_duration', true ); // badge duration
    $postdate = get_the_time( 'Y-m-d' ); // post date
    $postdatestamp = strtotime( $postdate ); // post date in unix timestamp
    $difference = round ((time() - $postdatestamp) / (24*60*60));  // difference in days between now and product's post date

    if ( !empty( $title ) && empty( $duration ) || !empty( $title ) && $difference <= $duration ){ // Check to see if there is a title and the product is still within the duration timeframe if specified
        $class = !empty( $class ) ? $class : '';
        echo '<span class="wc_simple_product_badge ' . $class . '">' . $title . '</span>';
    }
}

// Display the product badge on the single page

add_filter( 'woocommerce_single_product_image_html', 'wc_simple_product_badge_display_single' );    
function wc_simple_product_badge_display_single( $img_html ) {
    $title = get_post_meta( get_the_ID(), '_wc_simple_product_badge_title', true ); // badge title
    $class = get_post_meta( get_the_ID(), '_wc_simple_product_badge_class', true ); // badge class
    $duration = get_post_meta( get_the_ID(), '_wc_simple_product_badge_duration', true ); // badge duration
    $single_opt = get_post_meta( get_the_ID(), '_wc_simple_product_badge_single_page_option', true ); // badge on single page
    $postdate = get_the_time( 'Y-m-d' ); // post date
    $postdatestamp = strtotime( $postdate ); // post date in unix timestamp
    $difference = round ((time() - $postdatestamp) / (24*60*60));  // difference in days between now and product's post date

    if ( !empty( $title ) && empty( $duration ) && $single_opt === 'yes' || !empty( $title ) && $difference <= $duration && $single_opt === 'yes' ){ // Check to see if there is a title and the product is still within the duration timeframe ()if specified) and the checkbox is checked to show on single page view
        $class = !empty( $class ) ? $class : '';
        echo '<span class="wc_simple_product_badge ' . $class . '">' . $title . '</span>';
        return $img_html;
}

    elseif ( $single_opt === 'no' ) { // Check to see if the checkbox is unchecked to show on single page view
        return $img_html;
    }

}

Answer 1

Create a class to store the variables as private, internal members. Set up the variables when the post object is set up, that's the action the_post.

Then assign class methods as callbacks instead of separate functions.

Here is your code slightly reformatted:

The class

class WPSE_WC_Badge
{
    private $title = '';    
    private $class = '';    
    private $duration = '';    
    private $postdate = '';    
    private $postdatestamp = '';    
    private $difference = 0;

    public function __construct( \WP_Post $post )
    {
        $this->title         = get_post_meta( $post->ID, '_wc_simple_product_badge_title', TRUE ); // badge title
        $this->class         = get_post_meta( $post->ID, '_wc_simple_product_badge_class', TRUE ); // badge class
        $this->duration      = get_post_meta( $post->ID, '_wc_simple_product_badge_duration', TRUE ); // badge duration
        $this->postdate      = get_the_time( 'Y-m-d', $post ); // post date
        $this->postdatestamp = strtotime( $this->postdate ); // post date in unix timestamp
        // difference in days between now and product's post date
        $this->difference = round( ( time() - $this->postdatestamp ) / DAY_IN_SECONDS );
    }

    function loop()
    {
        if ( ! empty( $this->title ) && empty( $this->duration )
             || ! empty( $this->title ) && $this->difference <= $this->duration
        )
        { // Check to see if there is a title and the product is still within the duration timeframe if specified
            echo '<span class="wc_simple_product_badge ' . $this->class . '">' . $this->title . '</span>';
        }
    }

    function single( $img_html )
    {
        $single_opt = get_post_meta( get_the_ID(), '_wc_simple_product_badge_single_page_option', TRUE ); // badge on single page

        if ( ! empty ( $this->title ) && empty( $this->duration ) && $single_opt === 'yes'
             || ! empty( $this->title ) && $this->difference <= $this->duration && $single_opt === 'yes'
        )
        {
            echo '<span class="wc_simple_product_badge ' . $this->class . '">' . $this->title . '</span>';

            return $img_html;
        }

        return $img_html;
    }
}

And the registration

add_action( 'the_post', function( \WP_Post $post ) {

    $badge = new WPSE_WC_Badge( $post );

    // Display the product badge on the shop page
    add_action( 'woocommerce_after_shop_loop_item_title', [ $badge, 'loop' ], 30 );
    // Display the product badge on the single page
    add_filter( 'woocommerce_single_product_image_html', [ $badge, 'single' ] );
});

I haven't tested this, please take it just as starting point.

Answer 2

While Toscho's answer isn't wrong, using a class would circumvent the issue, the OP's original question is regarding the usage of globals and as such answers should be directly geared to that.

--- Using Globals ---

Any variable defined in the global scope (not inside a function or similar) is inherently a global. To access a global with in a function you must use the global keyword e.g. global $myglobal

Here is your code refactored to demonstrate:

<?php

$product_data = array();

// populating the global on a hook before the other two
add_action('woocommerce_before_single_product','populate_product_data');
function populate_product_data() {
    global $product_data;

    $postdate = get_the_time( 'Y-m-d' ); // post date
    $postdatestamp = strtotime($postdate);

    $product_data = array(
        'title'         => get_post_meta( get_the_ID(), '_wc_simple_product_badge_title', true ), // badge title
        'class'         => get_post_meta( get_the_ID(), '_wc_simple_product_badge_class', true ), // badge class
        'duration'      => get_post_meta( get_the_ID(), '_wc_simple_product_badge_duration', true ), // badge duration
        'single_opt'    => get_post_meta( get_the_ID(), '_wc_simple_product_badge_single_page_option', true ), // badge on single page
        'postdate'      => $postdate,
        'postdatestamp' => $postdatestamp, // post date in unix timestamp
        'difference'    => round ((time() - $postdatestamp) / (24*60*60)),  // difference in days between now and 
    );
}

add_action( 'woocommerce_after_shop_loop_item_title', 'wc_simple_product_badge_display_shop', 30 );
function wc_simple_product_badge_display_shop() {
    global $product_data;

    if ( !empty( $product_data['title'] ) && empty( $product_data['duration'] ) || !empty( $product_data['title'] ) && $product_data['difference'] <= $product_data['duration'] ){ // Check to see if there is a title and the product is still within the duration timeframe if specified
        $class = !empty( $class ) ? $class : '';
        echo '<span class="wc_simple_product_badge ' . $product_data['class'] . '">' . $product_data['title'] . '</span>';
    }
}

// Display the product badge on the single page

add_filter( 'woocommerce_single_product_image_html', 'wc_simple_product_badge_display_single' );    
function wc_simple_product_badge_display_single( $img_html ) {
    global $product_data;

    if ( !empty( $product_data['title'] ) && empty( $product_data['duration'] ) && $product_data['single_opt'] === 'yes' || !empty( $product_data['title'] ) && $product_data['difference'] <= $product_data['duration'] && $product_data['single_opt'] === 'yes' ){ // Check to see if there is a title and the product is still within the duration timeframe ()if specified) and the checkbox is checked to show on single page view
        $class = !empty( $product_data['title'] ) ? $product_data['title'] : '';
        echo '<span class="wc_simple_product_badge ' . $product_data['class'] . '">' . $product_data['title'] . '</span>';
        return $img_html;
}

    elseif ( $product_data['single_opt'] === 'no' ) { // Check to see if the checkbox is unchecked to show on single page view
        return $img_html;
    }

}