Passer can be used with avoir or être in Le Passé Composé... and changes meaning

Most verbs use either avoir or être as the auxiliary verb in Passé composé (or other compound tense)but passer uses both, depending on its grammatical usage* and what it means in the sentence.
 
*Grammaphile's Corner : the technical grammatical distinction between these cases is actually whether the verb is used in a transitive or intransitive manner. 
- The transitive version (the version with a direct object) uses avoir.
- The intransitive version (lacking a direct object), uses être.
 

être + passé [devant, par, chez, etc]

= pass by [something/somewhere]
= go past [something/somewhere]
= stop by [somewhere]
= pop by [somewhere]

Je suis passé par la maison en allant au travail.I passed by the house on the way to work.

Elle est passée chez Laurent hier.She passed by Laurent's place yesterday.

Est-elle passée par la pharmacie comme je lui ai demandé?Did she pop by the pharmacy as I asked her?

Nous sommes passés devant la poste.We went past the post office.

Note that in each case where être is the auxilliary, the verb passer is followed by a preposition (en, sur, dans, à etc.).  
In these cases passer is usually about passing by something, going past something, stopping or popping by somewhere.

(See also Agreeing past participle with subject's gender and number with (+ être) verbs in Le Passé Composé)
 

avoir + passé [quelque chose]

= spend [time]
= take [a test or exam] 
= pass [something] to [someone]

J'ai passé l'été dernier en Italie.I spent last summer in Italy.

J'ai passé mon examen hier.I took my exam yesterday.

Il a passé le sel à son père.He passed the salt to his father.

When passer is followed immediately by a noun (as opposed to a preposition), it uses avoir as the auxiliary, like most verbs.  
 
It can be very tricky to get the distinction here if you think in terms of what passer means in English (English verbs are very often 'prepositional', meaning we say things like to climb on a horse as well as mount a horse which are equivalent in meaning but grammatically very different - our verbs very often have prepositions where they don't in French!).
 

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Examples and resources

J'ai passé l'été dernier en Italie.I spent last summer in Italy.
Es-tu passé par la boulangerie?Did you stop by the bakery?
Il a passé le sel à son père.He passed the salt to his father.
On faisait la queue à la poste quand ce couple est passé devant tout le monde sans même s'excuser !We were queueing [US: waiting in line] at the post office when this couple jumped past [US: cut in front of] everyone without even saying sorry!
Je suis passé par la maison en allant au travail.I passed by the house on the way to work.
J'ai passé l'après-midi à me faire dorer la pilule.I spent the afternoon sunbathing.
Est-elle passée par la pharmacie comme je lui ai demandé?Did she pop by the pharmacy as I asked her?
Nous sommes passés devant la poste.We went past the post office.
J'ai passé mon examen hier.I took my exam yesterday.
Elle est passée chez Laurent hier.She passed by Laurent's place yesterday.
Thinking...