GravityView: Show the result of a calculation using a Custom Content field

gravityview-calculations-in-custom-content-field.php

<?php
/**
 * Referenced in this article: http://docs.gravityview.co/article/235-how-to-make-calculations-using-the-custom-content-field
 */

/** Modify the content returned from the Custom Content field */
add_filter( 'gravityview/fields/custom/content_before', 'my_gv_custom_content_before', 10, 2 );

/**
 * Replaces the %CALC% placeholder by the result of a calc operation between entries' fields
 *
 * @param  string $content Custom Content field content
 * @param  \GV\Template_Context $context
 * @return string
 */
function my_gv_custom_content_before( $content, $context = null ) {

	// this is the tag used inside the Custom Content Field
	$tag = '%CALC%';

	// Is the placeholder tag in the Custom Content field? If not, return original value.
	if( false === strpos( $content, $tag ) ) {
		return $content;
	}

	$replace = '';
	
	// If the fields used for the calculation are set, replace the tag with the calculation.
	if( isset( $context->entry['15'] ) && isset( $context->entry['37'] ) ) {
		$replace = $context->entry['15'] +  $context->entry['37'];
	}

	return str_replace( $tag, $replace, $content );
}