Entrer 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 entrer 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 + entré [en, dans, etc.]

= to go/come/get into  
= to enter

Je suis entré dans le salon sans que tu me voies.I walked into the living-room without you seeing me.

Quand elle est entrée dans la salle, tous les regards se sont tournés vers elle.When she came into the room, all eyes turned towards her.  
When she entered the room, all eyes turned towards her.

Mes filles sont entrées en CP cette année.My daughters started Primary School this year.
 

Notice that in each case where être is the auxilliary, the verb entrer is followed by a preposition (en, sur, dans, à etc.).  
So in these cases entrer is usually about getting into, coming into, going into or entering.

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

avoir + entré [quelque chose] dans [quelque chose]

= to enter/put [something] into [something]
Pour accéder à ce site, j'ai entré mon mot de passe secret.To access this website, I entered my secret password.
 
When entrer 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 entrer means in English (English verbs are very often 'prepositional', meaning we say things like to go into a room as well as enter a room which are equivalent in meaning but grammatically very different - our verbs very often have prepositions where they don't in French!).
   
Here is the list of all "two-auxiliaryverbs in compound tenses:

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Examples and resources

Mes filles sont entrées en CP cette année.My daughters started Primary School this year.
Pour accéder à ce site, j'ai entré mon mot de passe secret.To access this website, I entered my secret password.
Je suis entré dans le salon sans que tu me voies.I walked into the living-room without you seeing me.
Quand elle est entrée dans la salle, tous les regards se sont tournés vers elle.When she came into the room, all eyes turned towards her.  
When she entered the room, all eyes turned towards her.
Thinking...