When Arabic speakers type in Latin script, certain sounds that have no Latin equivalent get numbers: ع (a deep guttural vowel) becomes 3; ح (a breathed H) becomes 7; خ (the guttural kh) becomes 5 or x. So: habibi becomes 7abibi. Akhbar becomes a5bar. Khalas becomes xalas.

Entitely phonetic, readable instantly to anyone who knows the language. Arabizi developed in the era of early mobile phones where Arabic keyboards didn't exist, survived into WhatsApp, and shows no signs of going anywhere.