#vulnerable code 
                              @Override
  public void visitAssignmentExpression(AssignmentExpressionTree assignment) {
    SyntaxToken lastToken = ((PHPTree) assignment.variable()).getLastToken();
    String variableName = lastToken.text();
    checkVariable(lastToken, variableName, assignment.value());
    super.visitAssignmentExpression(assignment);
  } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public void visitAssignmentExpression(AssignmentExpressionTree assignment) {
    checkVariable(((PHPTree) assignment.variable()).getLastToken(), assignment.value());
    super.visitAssignmentExpression(assignment);
  }