What's wrong with the dozen languages that are obviously and manifestly superior to PHP in every respect?
> Starting them off with a fairly easy language that's widely widely widely used and easily introduces simple topics like variables, loops, etc is probably a terrible idea.
You're unqualified to be having this conversation. PHP is not a general purpose programming language, nor is it exemplary in any way whatsoever. It is a crippled, limited and broken language that is widely known among computer science professionals to be a pastiche of compromises.
For students, compare it to Python or Ruby, to name just two obviously superior classroom candidates that have every advantage over PHP. PHP can't even be tested or run without a server available. And I shouldn't have to tell you this.
http://www.securityfocus.com/news/11430
Quote: "Perhaps PHP should stand for Pretty Hard to Protect: A week after a prominent bug finder and developer left the PHP Group, data from the National Vulnerability Database has underscored the need for better security in PHP-based Web applications ... A search of the database, maintained by the National Institute of Standards and Technology (NIST), found that Web applications written in PHP likely account for 43 percent of the security issues found so far in 2006, up from 29 percent in 2005."