Yes, Pennsylvania is an at-will employment state. This means that, unless there is a contract stating otherwise, either the employer or employee can terminate the employment relationship at any time, for any reason, or for no reason at all, with some exceptions for unlawful discrimination or retaliation.
Under Pennsylvania law, an employment agreement will be recognized where both parties manifest a clear intent to be bound by its terms, the terms are sufficiently definite to be enforced, and the agreement is supported by consideration.
The Personnel Files Act of the Commonwealth of Pennsylvania states that you can look at your file once a year at a convenient time, and while you cannot make photocopies of the materials, you can take notes on what is in the files. External letters of reference are not available for you to inspect.
The short answer is yes, in most cases an employer can terminate an employee without providing a written warning. This practice, while potentially jarring for employees, is still legally permissible under California's employment laws.
Pennsylvania is an at-will employment state, which means that in the absence of a written employment or collective bargaining agreement, either the employer or the employee may terminate employment for any reason that is not contrary to law. Stumpp v. Stroudsburg Mun. Auth., 658 A.
Pennsylvania does not have a state law requiring employers to provide advance notice of termination. However, employers may be subject to the federal WARN Act, which requires some employers to provide advance notice of mass layoffs or plant closings.