To temporarily disable syntax highlighting in GNU nano, use the -Ynone option:
Instead of
nano myfile.php
use
nano -Ynone myfile.php
In order to permanently disable nano syntax highlighting, run this command:
echo "alias nano='nano -Ynone'" >> ~/.$(echo $SHELL | rev | cut -d/ -f1 | rev)rc source ~/.$(echo $SHELL | rev | cut -d/ -f1 | rev)rc # Reload immediately
This will add nano -Ynone as an alias for nano into your .bashrc or .zshrc