Add Donate Button

<?php
/**
 * USAGE:
 *
 * To display a particular campaign's button, just add this to your template:

ed_charitable_get_campaign_donate_button( 123 );

 * Replace 123 with the ID of your campaign.
 */

/**
 * Display a donate button for a specific campaign.
 *
 * Clicking on "Donate" will open the donation form modal directly
 * on the page.
 *
 * @param  int $campaign_id
 * @return string
 */
function ed_charitable_get_campaign_donate_button( $campaign_id ) {
	// Get the campaign.
	$campaign = charitable_get_campaign( $campaign_id );

	// Add the donate modal window to the footer. This is invisible until the button is clicked.
	add_action(
		'wp_footer',
		function() use ( $campaign ) {
			charitable_template( 'campaign/donate-modal-window.php', array( 'campaign' => $campaign ) );
		}
	);

	ob_start();

	// Render the donate button.
	charitable_template_donate_button( $campaign );

	// Load scripts that are required for the modal to work.
	Charitable_Public::get_instance()->enqueue_donation_form_scripts();

	return ob_get_clean();
}

/**
 * To add a shortcode to display a campaign's donate button, include this function below.
 *
 * @param  array $atts User-defined shortcode attributes.
 * @return string
 */
function ed_charitable_campaign_donate_button_shortcode( $atts ) {
	if ( ! array_key_exists( 'campaign_id', $atts ) ) {
		return '';
	}

	return ed_charitable_get_campaign_donate_button( $atts['campaign_id'] );
}

add_shortcode( 'charitable_donate_button', 'ed_charitable_campaign_donate_button_shortcode' );