PHP Fatal error: Array and string offset access syntax with curly braces is no longer supported

Symptoms

Your site throws internal server errors (500), and an error similar to the following can be seen in the PHP error log.

PHP Fatal error:  Array and string offset access syntax with curly braces is no longer supported in /home/cpusername/public_html/wp-includes/script-loader.php on line 707

Description

The support for accessing array elements and string offsets with curly braces was deprecated in PHP version 7.4 and removed in version 8.0.

Workaround

The curly braces in the site's code will have to be replaced with square braces.

Additional resources

PHP 7 ChangeLog: Version 7.4.0