gistfile1.php

<?php

class Hooks
{
	public function outputFrontendTemplate($buffer)
	{
		$buffer = preg_replace_callback(
			'~class="([^"]+)"~',
			function ($matches) {
				$classes = explode(' ', $matches[1]);
				$classes = array_filter($classes);

				$index = array_search('invisible', $classes);
				if ($index !== false) {
					$classes[$index] = 'sr-only';
				}

				return sprintf('class="%s"', implode(' ', $classes));
			},
			$buffer
		);

		return $buffer;
	}
}