PHP - How to check for a class

page.php

function the_thing() {
	$classes = get_body_class();
	if( in_array( 'the-class', $classes, true ) ) {
		// Do a thing
	}
}

// or

function the_thing() {
  if( class_exists( 'foo' ) ) {
    // Do a thing
  }
  else {
    // Do another thing
  }
}