What does Paul mean when he speaks of "in Christ"? Does Paul point to any type of mystical experience when he speaks of "in Christ"? What does he express? It seems this phrase cannot be taken literally - so what is the deeper meaning of it, possibly from a philological point of view?
Since there are tons of different places in which Paul uses the phrase, some specific references might help here.

In Acts 24:24, for example, Paul refers to "faith in Christ Jesus." Faith here refers not to the pseudo-power claimed by the "word of faith" movement, but rather having faith in the testimony of Christ.

Romans 6:3, on the other hand, refers to being "baptized into Christ." Here the Greek word baptizo literally means "immersed" or fully a part of Christ. It could also refer, in this context, to being baptized in the name of Christ.

Romans 8:1 provides another variation of the phrase: "There is now no condemnation for those who are in Christ Jesus." [emphasis mine] Perhaps this is the type of phrase you're looking for, as Paul uses this terminology quite a bit. The Complete Jewish Bible may provide a clarification in its translation: "Therefore, there is no longer any condemnation awaiting those who are in union with the Messiah Yeshua." [emphasis mine] (Yeshua is the commonly accepted Hebrew name translated into Greek and Latin as Iesus, or into English as Jesus.)

Galatians 2:20, while it may not use the phrase in all translations, may further explain the concept. "I have been crucified with Christ, and it is no longer I who live, but Christ lives in me. So the life I now live in the body, I live because of the faithfulness of the Son of God, who loved me and gave himself for me." To Paul his old life as Saul of Tarsus, persecutor of Christians, was gone. Saul was dead, crucified as Christ was. His new life through a full-bonded union with Christ was represented by his new name, Paul the apostle. His life was completely subsumed by the lift of Christ. Obviously, as @Narnian points out, this is not a physical movement into a place or person, but a metaphysical or spiritual replacement.