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PHP Ninja Manual
PHP Ninja Manual is another one of those great Web development resources that you didn't know you couldn't live without until you used it for the first time. This extension pops up definitions, documentation, and examples for PHP developers. It's nice, because you don't need to open a new tab, or go to a new page to look something up quickly.
How often used: When I'm writing PHP code, I use it a lot.
Live without it factor: I could live without it, but I'd prefer not too.