code: public function process( File $phpcsFile, $stackPtr ) {
  
  		$tokens = $phpcsFile->getTokens();
  
  		if ( '$_SESSION' === $tokens[ $stackPtr ]['content'] ) {
  			$phpcsFile->addError(
  				'Usage of $_SESSION variable is prohibited.',
  				$stackPtr,
  				'SessionVarsProhibited'
  			);
  		}
  	}
docs: Processes this test, when one of its tokens is encountered.
  
  @param \PHP_CodeSniffer\Files\File $phpcsFile The PHP_CodeSniffer file where the
  token was found.
  @param int                         $stackPtr  The position of the current token
  in the stack.
  
  @return void