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Initial protonation of the alkene double bond creates a secondary carbocation: $$\text{Ph}-\text{CH}_2-\text{CH}^+-\text{CH}_3$$.

This secondary carbocation undergoes a 1,2-hydride shift to form a more stable benzylic carbocation: $$\text{Ph}-\text{CH}^+-\text{CH}_2-\text{CH}_3$$. The benzylic carbocation is highly stable due to resonance with the benzene ring.

Nucleophilic attack by $$\text{Cl}^-$$ on this benzylic position yields 1-chloro-1-phenylpropane.