In science and math, one common complaint of professors at the
college level is that students are lacking in fundamentals. If
you are well grounded in the fundamentals, you can easily build
on them at a more advanced level.
As teachers, we're at an advanced level compared to our
students. We want to use that. However, while our students may
be able to get through the advanced material, they lack a
foundation. In high school we tend to provide a lot of help and
the students get through.
I'd say that php and mysql are quite advanced. I'd rather have
students understand more fundamental topics first: databases,
how the internet works, etc.