The khimar is "a headpiece ... which covers the hair, forehead, sides of the head, neck, shoulders, and chest," but not, at least in this instance, the face.

Philadelphia Police Department Directive 78 apparently prescribes a uniform uniform, with no exceptions for any religious apparel or any religious symbols. The case suggests that the uniform requirement is broad enough to exclude ashes on the forehead on Ash Wednesday, and therefore basically any non-uniform symbols.

Police officer Kimberlie Webb claimed that Title VII of the Civil Rights Act of 1964 required the city to accommodate her religious practice by exempting her from the strict uniformity requirement, and letting her wear the khimar. Title VII does require employers to provide exemptions for employees whose religions conflict with generally applicable work rules, but not when granting such an exemption would create an "undue hardship" for the employer. Courts have set the "undue hardship" bar pretty low, so that anything "more than a de minimus cost" would constitute an "undue hardship" that the employer need not bear.

The court held that requiring a religious exemption from Directive 78 would indeed create an "undue hardship":

The Directive's detailed standards with no accomodation for religious symbols and attire not only promote the need for uniformity, but also enhance cohesiveness, cooperation, and the esprit de corps of the police force. Prohibiting religious symbols and attire helps to prevent any divisiveness on the basis of religion both within the force itself and when it encounters the diverse population of Philadelphia.... Police Directive 78 is designed to maintain religious neutrality, but in this case in a para-military organization for the good not only of the police officers themselves but also of the public in general.

Thanks for How Appealing for the pointer.