Using the canonical function name for PHP functions is strongly recommended, as aliases may be deprecated or removed without (much) warning.
This replaces all uses of the following:
* `join()` with `implode()`
* `sizeof()` with `count()`
* `is_writeable()` with `is_writable()`
* `doubleval()` with a `(float)` cast
In part, this is a follow-up to #47746.
Props jrf.
See #50767.
git-svn-id: https://develop.svn.wordpress.org/trunk@49193 602fd350-edb4-49c9-b593-d223f7449a82