public FileSystemInterface::validScheme($scheme)
Checks that the scheme of a stream URI is valid.
Confirms that there is a registered stream handler for the provided scheme and that it is callable. This is useful if you want to confirm a valid scheme without creating a new instance of the registered handler.
string $scheme: A URI scheme, a stream is referenced as "scheme://target".
bool Returns TRUE if the string is the name of a validated stream, or FALSE if the scheme does not have a registered handler.
public function validScheme($scheme);
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!File!FileSystemInterface.php/function/FileSystemInterface::validScheme/8.1.x