MDL-84351 url: Add method for getting slashargument

.upgradenotes/MDL-84351-2025040804272401.yml

@@ -0,0 +1,5 @@
+issueNumber: MDL-84351
+notes:
+  core:
+    - message: The public method `get_slashargument` has been added to the `url` class.
+      type: improved

lib/classes/url.php

@@ -936,6 +936,17 @@ All values that are not arrays should be a string.');
     public function get_port() {
         return $this->port;
     }
+
+    /**
+     * Returns the 'slashargument' portion of a URL. For example, if the URL is
+     * http://www.example.org.com/pluginfile.php/1/core_admin/logocompact/ then this will
+     * return '1/core_admin/logocompact/'.
+     *
+     * @return string Slash argument as string.
+     */
+    public function get_slashargument(): string {
+        return $this->slashargument;
+    }
 }
 
 // Alias this class to the old name.

lib/tests/url_test.php

@@ -334,6 +334,19 @@ final class url_test extends \advanced_testcase {
         $this->assertMatchesRegularExpression($expected, $url->out(true));
     }
 
+    /**
+     * Test the get_slashargument method.
+     */
+    public function test_get_slashargument(): void {
+        $this->resetAfterTest();
+
+        $url = new url('/pluginfile.php/14/user/private/capybara.png');
+        $this->assertEquals('/14/user/private/capybara.png', $url->get_slashargument());
+
+        $url = new url('/image/capybara.png');
+        $this->assertEmpty($url->get_slashargument());
+    }
+
     /**
      * Data provider for make_pluginfile_url tests.
      *