General: Correct force_ssl_content() to always return a boolean value.

This aims to bring parity with `force_ssl_admin()`.

Includes:
* Allowing `force_ssl_content()` to properly accept `false` as a value.
* Correcting an erroneous `! $force` conditional that should have been reversed.
* Adding unit tests to confirm valid behavior.

Follow-up to [https://mu.trac.wordpress.org/changeset/1979 mu:1979], [11903], [12603], [47808], [59830].

Props justlevine for initial patch.
See #52217.

git-svn-id: https://develop.svn.wordpress.org/trunk@60147 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/ms-functions.php

@@ -2420,15 +2420,15 @@ function welcome_user_msg_filter( $text ) {
  *
  * @since 2.8.5
  *
- * @param bool $force
+ * @param bool|null $force Optional. Whether to force SSL in admin screens. Default null.
  * @return bool True if forced, false if not forced.
  */
-function force_ssl_content( $force = '' ) {
+function force_ssl_content( $force = null ) {
 	static $forced_content = false;
 
-	if ( ! $force ) {
+	if ( ! is_null( $force ) ) {
 		$old_forced     = $forced_content;
-		$forced_content = $force;
+		$forced_content = (bool) $force;
 		return $old_forced;
 	}
 

tests/phpunit/tests/multisite/forceSslContent.php

@@ -0,0 +1,53 @@
+<?php
+/**
+ * Test cases for the `force_ssl_content()` function.
+ *
+ * @since 6.9.0
+ *
+ * @group functions
+ * @group multisite
+ * @group ms-required
+ *
+ * @covers ::force_ssl_content
+ */
+class Tests_Functions_ForceSslContent extends WP_UnitTestCase {
+
+	public function set_up() {
+		parent::set_up();
+		// Reset the `$forced_content` static variable before each test.
+		force_ssl_content( false );
+	}
+
+	/**
+	 * Tests that force_ssl_content() returns expected values based on various inputs.
+	 *
+	 * @dataProvider data_force_ssl_content
+	 *
+	 * @param mixed $input    The input value to test.
+	 * @param bool  $expected The expected result for subsequent calls.
+	 */
+	public function test_force_ssl_content( $input, $expected ) {
+		// The first call always returns the previous value.
+		$this->assertFalse( force_ssl_content( $input ), 'First call did not return the expected value' );
+
+		// Call again to check subsequent behavior.
+		$this->assertSame( $expected, force_ssl_content( $input ), 'Subsequent call did not return the expected value' );
+	}
+
+	/**
+	 * Data provider for testing force_ssl_content().
+	 *
+	 * @return array[]
+	 */
+	public function data_force_ssl_content() {
+		return array(
+			'default'          => array( null, false ),
+			'true'             => array( true, true ),
+			'false'            => array( false, false ),
+			'non-empty string' => array( 'some string', true ),
+			'empty string'     => array( '', false ),
+			'integer 1'        => array( 1, true ),
+			'integer 0'        => array( 0, false ),
+		);
+	}
+}