i agree but it's not always that simple. sometimes you need PHP to do 5% of a job, and the rest is LAMP or other. in which case I'd learn PHP even if it's not my thing, so i could get the job that pays the bills. you can't always have the whole cake.
PHP is pretty old, one of the original languages to add dynamic content to html. it was not very popular until a couple of things happened: a whole lot of extensions (including lots of http and database handling) became bundled in standard PHP, and some people wrote mediawiki and drupal etc, which wikipedia and other high exposure projects exemplified. PHP also only fairly recently became part of standard install for unix and linux variants.
perl community confusion with the version 5 to version 6 debacle, has also helped PHP proliferate in favor of perl.