Those are two completely different things. Slang is only used in an informal setting (e.g. dialect), phrasal verbs are also part of formal English.
Let me explain.
Slang is a type of language consisting of words and phrases that are regarded as very informal, are more common in speech than writing, and are typically restricted to a particular context or group of people. Think dialects or specific language used among youngsters, for example.
Example 1: "grass" is slang for marijuana
Example 2: "the nick" is British slang for prison
Unlike slang, a phrasal verb is used in formal language. A phrasal verb is a verb that is made up of a main verb together with an adverb or a preposition, or both. Typically, their meaning is not obvious from the meanings of the individual words themselves.
That specific group of words form a meaning of their own.
A transitive phrasal verb only makes sense if connected to an object. An intransitive one will also make sense without an object attached to it.
Examples of intransitive phrasal verbs:
Break up (= couple that separates): "We broke up two years ago."
Stand off (= move or keep away): "The women wood off at a slight distance"
Examples of transitive phrasal verbs:
Break up something (with object, different meaning): "The police were called to break up the fight."
To pull down something (to demolish): "They pulled the house down and redeveloped the site."
I hope this clears things up a bit? If still in doubt, let need know.