I looked at cakephp, symfony, CodeIgniter, Kohana, Yii, Akelos and some more.
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From the above CakePHP is well documented and fairly easy to learn. Of the shortcomings can be noted support for PHP4 and not very progressive community, albeit large.
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Yii — cool stuff written using PHP5 and fashion design patterns, moreover, tightly integrated with chips web2.0 (AJAX). Good documentation of the APIs, but almost no examples.
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Akelos is too crude.
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CodeIgniter is php4, weak ORM.
Kohana (a fork from CodeIgniter) — php5, poorly documented, weak ORM.
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Symfony — trying to make a clone RubyOnRails php. However it is more difficult to learn than the original and offers less opportunities. IMHO better then rails, or learn Django.
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The result:
for beginners cakephp, yii advanced