PhpStorm has two types of autocompletion: structural completion and word expansion. Structural autocompletion makes predictions based on its understanding of PHP, while the latter tries to predict the word currently being typed based on previously typed words. Word expansion also works in comments and docstrings and it's similar to vim's omnicompletion. Both types of autocompletion work extremely well, have little to no problems and are quite fast even when loading suggestions on the go.
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Pro First-class mouse support
Slap supports mouse keybindings even though it works inside the terminal and also through SSH.
Pro Uses desktop-like keybindings
Slap features configurable keybindings (Ctrl+S, Ctrl+Z, etc).
Pro Great terminal interface
Pro Intelligent code completion
PhpStorm has two types of autocompletion: structural completion and word expansion.
Structural autocompletion makes predictions based on its understanding of PHP, while the latter tries to predict the word currently being typed based on previously typed words. Word expansion also works in comments and docstrings and it's similar to vim's omnicompletion.
Both types of autocompletion work extremely well, have little to no problems and are quite fast even when loading suggestions on the go.
Pro Supports various PHP frameworks
Supports Symfony2 (including Twig and Doctrine), Yii frameworks, and Laravel.
Pro PHPDoc support
PHPDoc is a documentation generator. It allows automatically generating documentation from specifically formatted comments.
Pro Inspections of all kinds
Pro External command support
Ability to setup custom external commands, which is something inherited from eclipse and found lacking on netbeans.
Pro Database view
The database tab, while not perfect, provides all the tools you need for daily usage. You always have your DB overview on the side (you can hide it when not needed).
Pro Phar package recognition
Phar support allows running complete applications out of .phar files.
Pro Built-in debugger
Has built in support for Xdebug including remote debugging.
Pro Auto-sync with remote folders
It allows monitoring code and sync the files that need to be changed with a remote server. This works more efficiently than libnotify and faster than vagrant auto rsync.
Pro Plugins allow futureproofing and customization
Active development work on plugins is always a plus, as it can extend the use of the IDE.
Pro Has built-in console and SSH clients
This allows execution of needed commands without switching to other windows. It also keeps track of what's happening right in the IDE, which is especially good with Vagrant, because you can connect to a VM in a single click.
Pro Integrated Git GUI support
Allows users to manage their repositories directly from the IDE with a GUI which lets you do all Git commands.
Pro Integrated Vagrant support
It recognizes Vagrantfile and allows full control from the IDE.
Pro Zero latency typing
Type codes with pleasure. Delay of visual feedback on a computer display has an important effect on typist's behavior and satisfaction, as cited here.
Pro It comes with Testing RESTful Web Services
Save time of building your own or integrating libraries for testing RESTful Web Services. PHPStorm enables you to test endpoints without leaving your workspace or disrupting your workflow . You can code your endpoints in one window and test it immediately from another window.
Pro Lots of plugins
Lots of plugins from JetBrains and the community are available, from useful things like support for Docker, Vagrant, Angular, Vue.js and more useless (but fun) like a nyan cat progress bar.
Pro Emmet integrated
Pro Templating support
Supports Twig and Laravel's Blade (among others). Technically the later is part of a framework which Phpstorm supports but I've seen IDEs (e.g. Netbeans) with Laravel support but no blade support so it does deserve a notable mention.
Pro Responsive core developers
Pro Task/Context functionality
Similar to Eclipse Mylyn: you can create tasks or retrieve them from your ticketing system, and each task keeps its own context (opened files, modifications, etc.).
Pro Code Refactoring
Quickly rename classes, methods, and variables used across multiple files in the project.
Pro Regular updates for fixes and new features
PhpStorm is clearly being developed continually, which gives confidence that it will be available for some time and is not abandonware.
Con Requires node.js
Con CLI only
Slap only runs through the terminal.
Con Resource exhaustive
It often maxes out even 4 cores, not to mention several hundred MBs of RSS with a single small project opened.
It is not free software (free as in freedom).
Currently costs £78 but does not seem to offer much more than some free counterparts.
Con Slow performance
A very slow indexing, many memory leaks, large projects after some time will be very slow, opening large files like 10mb can even break IDE.
Con Interface is cluttered
Con Sometimes buggy
Index gets corrupt which results in errors in syntax presentation, code validation and auto-completion. Version-control system sometimes stops working, occasionally freezes in big files