Changed get_template_directory to get_stylesheet_directory to facilitate child themes

acf-auto-export.php

<?php
/**
 * Automated PHP export for Advanced Custom Fields
 *
 * Export is initiated whenever an admin publishes a new field group 
 * or saves changes to an existing field group.
 *
 * Place this code in your theme's functions.php file.
 *
 */
add_action('admin_head-post.php', 'my_acf_export');
function my_acf_export() {  
	global $acf;

	// IMPORTANT: 
	// Enter the absolute path to the file you want to export into:
	$file = get_stylesheet_directory().'/inc/fields.php';

	// Only continue if we're saving a field group
	if ( empty($_GET['message']) || ($_GET['message'] != '1' && $_GET['message'] != '6') ) {
		return;
	}

	$fields = array();
	$fields = get_posts(array(
		'numberposts' 	=> -1,
		'post_type' 	=> 'acf',
		'orderby' 		=> 'menu_order title',
		'order' 		=> 'asc',
	));

	if($fields)
	{
				
$output = '
	if(function_exists("register_field_group"))
	{';

		foreach($fields as $field)
		{
			$var = array(
				'id' => uniqid(),
				'title' => get_the_title($field->ID),
				'fields' => $acf->get_acf_fields($field->ID),
				'location' => $acf->get_acf_location($field->ID),
				'options' => $acf->get_acf_options($field->ID),
				'menu_order' => $field->menu_order,
			);
			
			$html = var_export($var, true);
			
			// change double spaces to tabs
			$html = str_replace("  ", "\t", $html);
			
			// add extra tab at start of each line
			$html = str_replace("\n", "\n\t", $html);

			// remove excess space from beginning of arrays
			$html = str_replace("array (", "array(", $html);
			
			$output .= ' 
		register_field_group('.$html.'); ';

		}

	$output .= '
	}
	';
	}
	else
	{
		$output = __('No field groups were selected','acf');
	}


	if (is_writable($file)) {
		file_put_contents($file, $output);
		define('ACF_EXPORT_FILE', $file);
		add_action('admin_notices', 'my_acf_export_success');
	}  else {
		add_action('admin_notices', 'my_acf_export_fail');
	}
} 

/**
 * Notice to display when export is completed successfully
 */
function my_acf_export_success( $file = '' ) {
	if (defined('ACF_EXPORT_FILE')){
		$destination = ' to <strong>'.ACF_EXPORT_FILE.'</strong>';
	} else {
		$destination = '';
	}
	echo '<div class="updated"><p>All fields successfully exported'.$destination.'.</p></div>';
}

/**
 * Error message to display when export fails (file not writable)
 */
function my_acf_export_fail( $file = '' ) {
	echo '<div class="error"><p><strong>Automated Export Error:</strong> The export file you\'ve specified is not writeable.</p></div>';
}