Gyaru-moji

In fact, newspaper accounts chronicle the rise of gyaru-moji (“girl-signs”), a “secret” texting language of symbols improvised by Japanese teenage girls.

Source: wiktionary

Today, the same kind of inventiveness is seen in SMS gyarumoji ('girl-talk'), which is a mixture of Japanese syllables, numerals, emoticons, and other characters.

Source: wiktionary

Gyarumoji (gal characters) are designed to hinder the decoding of text by those not in the know. They usually take the form of an unusual mixture of scripts (e.g. Japanese, the alphabet, Roman numerals, the Greek alphabet) with typographic, mathematical, and other symbols

Source: wiktionary

For example, the phonetic symbol に (ni) can be written in gyaru moji as |=, I=, (=, or L=

Source: wiktionary