The letter ب (Bay) is common in Urdu and Arabic. It is alike English letter 'B'.
Usage in Urdu: Behn (sister), Bayta (son), Bayti (daughter)
Usage in Arabic: Baab (door), Bayt (house), Baqara (cow)
Usage in English: Black, Back, Best
The letter پ (Pay) does not exist in Arabic. In English its counterpart is letter 'P'.
Usage in Urdu: Pakistan, Punkha (fan), Pani (water), Pooda (plant)
Usage in English: Password, Petrol, Past, Pink, Payment
Native Arabic speakers find it difficult to pronounce پ (Pay) = 'P'. So, they usually use ب (Bay) = 'B' instead of 'P'.
Example: Bakistan instead of Pakistan