Below, I list what I think the advantages are that one language has over the others. It should be noted that very often these advantages only apply within specific contexts.

What PHP has over Python and Ruby:

1. HTML templating at the core of the language.
2. The entire library is available without needing to import anything.
3. Simple Deployment

What Python has over Ruby:

1. Less syntax, cleaner code
2. A collection of simple functions becomes an object-oriented API without any work
3. Use of sugar and magic is kept in check, resulting in more consistent readability

What Ruby has over Python:

1. Library consistently uses object-oriented syntax, rather than a hybrid function/object approach.
2. How closures solve the list processing problem
3. Interpolate code within strings
4. Sugar and magic

What JavaScript has over Python and Ruby:

1. How closures solve the code organization problem
2. Can simulate any flavor of object-oriented programming
3. Concise object notation (JSON)
4. The dictionary and object types have been merged into a single type

Some interesting observations about the above:

1. Traditionally, PHP and JavaScript are web glue languages, and they both merge two common data types that are usually separate.
2. The fact that JavaScript has full-featured closures and can simulate any flavor of OOP is a profoundly powerful concept, given that the language is considered merely a scripting language.