Fatal error: Uncaught Error: [] operator not supported for strings - PHP 7.1

I'll use Joomla as an example in this short post. The same error and fix may apply for other CMS systems like WordPress and Drupal.

First you have to enable Joomla Debug modus, that'll show you where the error is happening. Open up configuration.php and set public $debug to 1.

If you refresh the page now, you'll see Joomla's error page, telling you about Fatal error: Uncaught Error:

Fatal error: Uncaught Error: [] operator not supported for strings D:\www\example.com\www\components\com\_layer\_slider\base\includes\slider\_markup\_init.php:98

OK, so we now know where the error is, it's on line 98 in com_layer_slider's base\includes\slider_markup_init.php file. In this case, the failing code is:

$data = '<script type="text/javascript">' . NL;

You can fix this PHP error by first initializing $data[] as an array:

$data = [];
$data[] = '<script type="text/javascript">' . NL;

A second method is to use the older style using array():

$data = array();
$data[] = '<script type="text/javascript">' . NL;

Let's hope not too many Joomla components, plugins and extensions use code like this. Not to mention other CMS systems like Expression Engine, WordPress, BuddyPress, PrestaShop...

Update 3-3-2018:
WordPress premium Betheme theme by Muffin group needs a fix in the file

wp-content\themes\betheme\functions\builder\front.php

On line 150:

// $section_style         = '';
$section_style         = [];

WordPress Revolution Slider

Older versions of WordPress plugin Revolution Slider by ThemePunch has this same deprecated PHP code, and throws an error:

Uncaught Error: [] operator not supported for strings in wp-content\plugins\revslider\includes\framework\base-admin.class.php:69

To fix this, add a line before line 69:

self::$arrMetaBoxes = []; # add this line to create the variable as an array
self::$arrMetaBoxes[] = $box;