Why is PHP considered to be the easiest web programming language to learn?

PHP is native to the web. While Ruby and Python have much cleaner syntax, more elegance, and more power, there will always be a layer of abstraction between Ruby/Python and the web itself -- after all, they were designed for much wider domains than the web.

Newbies to programming are typically newbies to sysadmin, and getting to Hello World in a Rails or Django is pretty painful -- for some even prohibitively so -- compared to PHP.

For newbies, it's easy to conceptualize that typing in:

http://mysite.com/something.php

...will execute the code stored in the file:

/path/to/mysite's/webroot/something.php

This simple one-to-one routing also mirrors that of HTML and other static files.

Beware, however, because this one-to-one routing also leads to security problems (i.e. people tend to keep all of their executable code within the webroot -- even secure code, which may contain passwords, hash salts, and other Privacy-Important code). Combine this with a lack of sysadmin experience, and many sites on the web are a chmod away from being totally exposed.

Responsible PHP like Symfony helps people avoid this, but Symfony requires the same level of sysadmin chops as Rails and Django.

Object oriented programming is optional

PHP is forgiving
The script continues running on minor faults.
When E_NOTICE (or even E_WARNINGs) are suppressed, the errors aren't even noticeable.

But also in the small things like substr: In C# you'll get a big fat exception when you'll try substr($text, 3) on a $text with 1 character.

Great online manual
http://php.net/manual/

Quick and Dirty is the default
The language is filled with useful shortcuts.
PHP lets me express what I want without typing an essay.