Code Modernization: Rename the readonly() function to wp_readonly().

Since PHP 8.1, `readonly` is a reserved keyword and cannot be used as a function name.

In order to avoid PHP parser errors, the `readonly()` function was extracted to a separate file and is now only included conditionally on PHP < 8.1.

This commit also:
* Moves the tests for the `__checked_selected_helper()` function and all the related functions to their own file.
* Switches to named data providers. This makes the output when using the `--testdox` option more descriptive and is helpful when trying to debug which data set from a data provider failed the test.
* Improves the tests in question to make them feature-complete and expand test coverage.

Props jrf, ayeshrajans, haosun, knutsp, swissspidy, SergeyBiryukov.
Fixes #53858.

git-svn-id: https://develop.svn.wordpress.org/trunk@51586 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/general-template.php

@@ -4813,17 +4813,27 @@ function disabled( $disabled, $current = true, $echo = true ) {
  *
  * Compares the first two arguments and if identical marks as readonly
  *
- * @since 4.9.0
+ * @since 5.9.0
  *
  * @param mixed $readonly One of the values to compare
  * @param mixed $current  (true) The other value to compare if not just true
  * @param bool  $echo     Whether to echo or just return the string
  * @return string HTML attribute or empty string
  */
-function readonly( $readonly, $current = true, $echo = true ) {
+function wp_readonly( $readonly, $current = true, $echo = true ) {
 	return __checked_selected_helper( $readonly, $current, $echo, 'readonly' );
 }
 
+/*
+ * Include a compat `readonly()` function on PHP < 8.1. Since PHP 8.1,
+ * `readonly` is a reserved keyword and cannot be used as a function name.
+ * In order to avoid PHP parser errors, this function was extracted
+ * to a separate file and is only included conditionally on PHP < 8.1.
+ */
+if ( PHP_VERSION_ID < 80100 ) {
+	require_once __DIR__ . '/php-compat/readonly.php';
+}
+
 /**
  * Private helper function for checked, selected, disabled and readonly.
  *