Adds support for `if truthy`, `if falsy`, `if equals` and `if not equals` to Handlebars PHP - https://github.com/salesforce/handlebars-php

class-handlebars-helpers.php

<?php
/**
 * Adds helpers to Handlebars to support the following expressions:
 * 
 * If truthy - `{{#if_truthy varName}}...{{/if_truthy}}`
 * If falsy - `{{#if_falsy varName}}...{{/if_falsy}}`
 * If equal - `{{#if_equal varName anotherVar}}...{{/if_equal}}`
 * If not equal - `{{#if_not_equal varName anotherVar}}...{{/if_not_equal}}`
 *
 * Supports the PHP implementation of Handlebars found here: https://github.com/salesforce/handlebars-php
 */
Class Handlebars_Helpers {
	private $helpers = array(
		'if_equal',
		'if_not_equal',
		'if_truthy',
		'if_falsy',
	);
	public function register( $handlebars ) {
		foreach( $this->helpers as $helper ) {
			$handlebars->addHelper( $helper, array( $this, 'helper_' . $helper ) );
		}
	}
	private function render_if( $template, $context ) {
		$template->setStopToken( 'else' );
		$buffer = $template->render( $context );
		$template->setStopToken( false );
		$template->discard();
		return $buffer;
	}
	private function render_else( $template, $context ) {
		$template->setStopToken( 'else' );
		$template->discard();
		$template->setStopToken( false );
		return $template->render( $context );
	}
	private function get_args_values( $args, $context ) {
		$args_arr = explode( ' ', $args );
		$args_arr = array_map( 'trim', $args_arr );

		$args_values = array();
		foreach ( $args_arr as $arg ) {
			$new_arg = '';
			if ( $this->is_args_string( $arg ) ) {
				$new_arg = trim( $arg, '"' );
			} else {
				$new_arg = $context->get( $arg );
			}
			$args_values[] = $new_arg;
		}
		return $args_values;
	}
	private function is_args_string( $arg ) {
		// Args in quotes are strings.
		// TODO - might need to support single quotes?
		if ( substr( $arg, 0, 1 ) === '"' && substr( $arg, -1 ) === '"' ) {
			return true;
		}
		return false;
	}
	public function helper_if_equal( $template, $context, $args, $source ) {
		$args_arr = $this->get_args_values( $args, $context );
		if ( count( $args_arr ) === 2 && $args_arr[0] === $args_arr[1] ) {
			return $this->render_if( $template, $context );
		} else {
			return $this->render_else( $template, $context );
		}
	}
	public function helper_if_not_equal( $template, $context, $args, $source ) {
		$args_arr = $this->get_args_values( $args, $context );

		if ( count( $args_arr ) === 2 && $args_arr[0] !== $args_arr[1] ) {
			return $this->render_if( $template, $context );
		} else {
			return $this->render_else( $template, $context );
		}
	}
	public function helper_if_truthy( $template, $context, $args, $source ) {
		$args_arr = $this->get_args_values( $args, $context );

		if ( count( $args_arr ) === 1 && $args_arr[0] ) {
			return $this->render_if( $template, $context );
		} else {
			return $this->render_else( $template, $context );
		}
	}
	public function helper_if_falsy( $template, $context, $args, $source ) {
		$args_arr = $this->get_args_values( $args, $context );

		if ( count( $args_arr ) === 1 && ! $args_arr[0] ) {
			return $this->render_if( $template, $context );
		} else {
			return $this->render_else( $template, $context );
		}
	}
}

handlebars-add-helper-class.php

<?php
/**
 * How to init and add the helpers to handlebars.
 */
require_once 'class-handlebars-helpers.php'

// Start with the default implementation of the Handlbars class as found in the repo:
$partials_dir    = '/path/to/partials';
$partials_loader = new FilesystemLoader( $partials_dir, array( 'extension' => 'html' ) );

// Init handlebars.
$handlebars = new Handlebars(
	array(
		'loader'          => $partials_loader,
		'partials_loader' => $partials_loader,
	)
);

// Now register the helpers
$helpers = new Handlebars_Helpers();
$helpers->register( $handlebars );