When comparing PHP vs Phalcon, the Slant community recommends Phalcon for most people. In the question“What are the best backend web frameworks?” Phalcon is ranked 10th while PHP is ranked 52nd. The most important reason people chose Phalcon is:
Since the framework is and extension built in C it's extremely fast and efficient. It's actually one of the fastest PHP frameworks, and according to some benchmarks it's the fastest framework out there.
Specs
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Pros
Pro One of the most common languages
According to the 2015 Stack Overflow Developer Survey (26,086 people surveyed), PHP was the 5th most popular/used language at 29.7%.
Pro Lots of tutorials online
Pro Used by most common CMS platforms
Many clients are looking for an easy-to-update web site that's flexible and free. Drupal and Wordpress fill those needs very well.
Pro Most prominent language for web applications
Part of the de facto standard web application stack.
Pro Great third-party package manager
PHP standard library is somewhat subpar, but if you need plugins, language features, composer has them all( you can even puzzle together a custom framework from composer).
Pro Fast
Since 7.x was released, PHP has become a pretty fast language.
Pro Lots of PHP frameworks available which help with development
PHP people love frameworks, and with frameworks such as Laravel, you can build a web app or API really fast (Facades, ORMs, scaffolding etc.)
Pro Great documentation
Pro Extremely fast
Since the framework is and extension built in C it's extremely fast and efficient. It's actually one of the fastest PHP frameworks, and according to some benchmarks it's the fastest framework out there.
Pro Open source
Phalcon is open source and is available under the BSD License.
Pro Uses Volt template engine
The Volt template engine, which is embedded into Phalcon itself takes it's inspiration from the Jinja template engine and as such it's nice to look at, with a clear and understandable syntax.
Volt also compiles very fast, like Phalcon itself, so it avoids being a bottleneck for the framework's overall speed.
Pro Very flexible project structure
You can set up the base project as you want. It's very flexible
Pro Clean and light project code
Since the framework code is not in the project directory, the code is light and clean.
Pro Customizable with Zephir
Zephir is a high-level language designed to create PHP extensions easily by PHP programmers with no knowledge in C.
Zephir does this by compiling directly to C and then the C program is in turn compiled to be run as a PHP extension. This, coupled with the fact that Zephir's syntax is very similar to PHP makes it a perfect way for PHP developers to use it for customizing Phalcon.
Pro Loosely coupled components
Some components can be used as standalone packages like models, views, etc..
Cons
Con Poorly designed language
Despite its widespread use, PHP is generally looked upon poorly from a design point of view. The consistency of function names and function argument order, lazily and borderline non-functional implementation of object oriented programming, can only receive requests via POST methods, slow version adoption (the PHP you learn right now may not work on every webserver you'll work on), and a focus on "hacking things together" rather than "doing it right". These are all very common complaints when it comes to working with PHP. While not a bad language to learn, PHP is not at all a good language to learn first, as it will probably teach bad habits.
Con Immense catalog of insecure frameworks
The most serious security problems in websites on the web today are almost universally found in popular PHP frameworks, CMS platforms, libraries and code samples, almost all stemming from poor language design, bad tutorials and awful resources.
Con After python, probably one of the worst languages ever
Con Poorly designed language, awful syntax & luckily on the decline
Nobody in their right mind is using PHP for new software, if you decide to learn it as your first language you'll be stuck working in teams with old developers who have had no interest in the computer programming field since they landed their first job while maintaining some 2000 era archaic website codebase.
Con Most tutorials are out of date
A lot of very bad tutorials are still widely circulated among beginners, and these tutorials teach very poor programming practices.
Con Most resources are poorly-written
Few resources exemplify the "correct" or secure use of features.
Con Interpreter being too permissive
If you forget the dollar sign, the variable name will be converted to a string.
Con Not for shared hosting
Phalcon needs root access to install the PHP extension which is written in C. Developers who plan on using Phalcon must use VPS or Cloud Hosting with root access available.
Con Require good programming skills
Not so easy to use if you want to gain the best from it.
Con Debugging requires knowledge of C
You need to be a C programmer to debug Zephir or C code. Or if Phalcon is not maintained anymore and you have a problem and don't have much skills in C, you will be hard-pressed to find a C programmer to fix it.