Sadly, the LAMP stack has reached canonical status. To alter any portion of it is to depart from the standard and embark on a never-ending uphill hike.

For example, nearly every Drupal module is developed against MySQL and then ported to PostgreSQL as an afterthought. Half of them have nothing but a big //FIXME where the PostgreSQL CREATE TABLE statements are supposed to be. And Drupal has actually made a big, occasionally controversial effort to remain DB-agnostic, to create and use an abstract database API, and to explicitly require every bit of core code to work and be tested under PostgreSQL. I'd guess that a lot of other PHP projects are far more tightly bound to MySQL.

Having said that, this article gives me fond thoughts of ripping MySQL off my servers and running aptitude install postgres.