Secure? Stable? Small? Speedy? Yes, it is!
(Yii 4S)
I was hoping that it was implied by the light weight part, but light weight makes it sound like its not a full stack framework. I really wanted to use Mature instead of stable because Mature has a more positive connotation than stable (To me anyways), but I didn’t like the fact it rhymes, the alliteration of Secure and Stable makes it a little more catchy without overdoing it. I’m looking at you Onman 
Claims about performance, security, reliability, professionalism etc. just invite debate. I say avoid that.
So you can either say some obviously irrefutable fact, which is boring, or make some outlandish subjective claim that it would be meaningless to argue with—which I recommend.
Now, what’s the key difference between Yii and other PHP MVC frameworks from the point of view of the programmer? In my view, it’s that it liberates me. Unlike any other PHP MVC, I feel that Yii is supporting me in implementing my architecture and design, not constraining me and imposing its architecture and design on me. I don’t know how to turn that into a tag line but to sell Yii to a programmer, I’d go with that.
Yii - sweet development
Yii - Rapid Application Development
Yii - Enterprise MVC Framework
Yii - Just do it
Lawsuit is imminent 
other idea would be Yii - Just code it.
Convention where it’s needed, configuration where it’s not?
Productivity = Yii + You
A fast, solid and secure PHP framework that works with you, not against you.
ok let me think:
Elastic features, resilient and secure framework, as easy as drinking water, Yii that is!
no technical terms
contains enterprise phrases such as elastic, resilient and secure to emphasize Yii can be used for enterprise applications
highlights the simplicity of usage by comparing it with drinking water. Simplicity is genius.
Yii that is little marketing trick here.
I have to look them up in a dictionary … 
I agree they are not phrases we use in our day to day life but if we are going to brand the framework we should use more sophisticated terms to highlight that it is an enterprise framework thus we will attract companies to start using Yii for their business needs hence we’ll be able to find jobs where you get to play with your favorite Yii framework at your office desk and get paid for it. Wouldn’t that be sweet?
Yii - easy and effective
Just make it “PHP framework” and be done with it. Or have no tagline or whatever they’re called at all. People know what it is anyway, they don’t need something that looks like we’re trying to persuade them.
Agree with rawtaz that this should be simple, but don’t underestimate the need for something that sums it up. Even developers are vulnerable to marketing:
"Yii - the smarter, better, faster PHP framework"
Personally, I don’t like explaining the technology (‘PHP framework’) and I don’t like excessive superlatives like ‘the best’, ‘easy’, ‘stable’, etc. We should have an easy, readable tagline, that makes sense even for non-technical people.
Something like:
Get it done easily.
Build the web.
More work, less time. (this one looks too much like jQuery, lol)
Create anything!
etc.
The main thing is that you shouldn’t explain what it is, you should express a feeling. Large brands don’t need to explain themselves, you should give the impression that everyone already knows what it is, and if you don’t, you should look it up.
When creating this thread I used the term “description” when in fact “mission” would have been be more appropriate and because of that we are mixing up slogans with mission statements here. A slogan is super short like Nike’s “just do it” etc. which is a whole other story but the mission statement is more about the whole idea behind Yii. What’s the purpose of Yii in the first place? It frees you from having to do the dirty, boring, code monkey work and enables you to focus on the creative aspects of coding instead. You are solving problems elegantly without reinventing the wheel over and over again and can rely on a solid codebase and community that backs you up in terms of security and ease of code reuse. That’s something I am looking for. It is not so much about the technology used but the overall ecosystem of a framework that catches my interest.
EDIT: fixing nerd typo “caches my interest”
Yii - The Chuck Norris of PHP frameworks
I think the Yii front page should be 100% free from marketing speech - no buzzwords, just the facts: what Yii can do for you, and nothing more.
Even the list of features should be carefully made to exclude any fluff and features made by marketing departments.
We dont’t have marketing departments 
I think "high-performance" is a key word for Yii and for its success. How many people have downloaded and tried Yii after reading this page (linked from the home page)?
And most important, they experienced that the claim is credible. That was good marketing, even if Yii does not have a marketing department.