As far as the usage of a/an is concerned, you should follow the general rule: 'a' should go before consonant, 'an' should go before a vowel.
So, in the case you mention, if you are speaking of an EU (see I used 'an', not 'a'. Do you guess why?) summit, where the PMs (Prime Ministers or equivalents) meet, you should use "a PM" (PM reads /pi-em/) because PM reads as starting with a consonant. On the other hand, if you reverse it to MP (Member of Parliament, which reads /em-pi/), you should say "an MP" because MP reads as starting with a vowel.
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