Standard Khmer phonetically reflects the orthography. Phnom Penh Dialect; however, deviates in a predictive way.


The “R” is a distinctive feature in Phnom Penh dialect, and is often the shibboleth of modern spoken Khmer. 
In standard Khmer the “R” is pronounced as a alveolar trill /r/ in word initial and in consonant clusters (although not in word final position).
In Phnom Penh Khmer, the “R” is realized in different iterations: as a uvular trill /ʀ/ or more popularly as a voiced glottal fricative /ɦ/ in word initial positions. However, in a consonant cluster where the “R” is adjacent to the nucleus and stressed, 5 things occur:

• “R” is completely assimilated
• Nucleus takes on breathy voice
• Drop in tone
• Open vowels close slightly
• Diphthongization in monophthong open vowels

This can be best exemplified in the word for “much”

Standard Khmer /craɜn/

Phnom Penh Khmer /cɘ̤ːn/

Source:
Filippi, Jean-Michel, and Hiep Chan Vicheth. “Khmer Pronouncing Dictionary Standard and Phnom Penh Dialect.” UNESCO Office Phnom Penh, 2016, unesdoc.unesco.org/Ulis/cgi-bin/ulis.pl?catno=246360&set=00581EBB0D_1_431&gp=0&lin=1&ll=3.