About Psalm

Psalm is a static analysis tool that attempts to dig into your program and find as many type-related bugs as possible.

It has a few features that go further than other similar tools:

Mixed type warnings
If Psalm cannot infer a type for an expression then it uses a mixed placeholder. Any mixed type is a sign of an insufficiently-documented codebase. You can configure Psalm warn when encountering mixed types by adding totallyTyped="true" attribute to your XML config file.
Logic checks
Psalm keeps track of logical assertions made about your code, so if ($a && $a) {} and if ($a && !$a) {} are both treated as issues. Psalm also keeps track of logical assertions made in prior code paths, preventing issues like if ($a) {} elseif ($a) {}.
Property initialisation checks
Psalm checks that all properties of a given object have values after the constructor is called.
Support for complicated array shapes
Psalm has support for object-like arrays, allowing you to specify types for all keys of an array if you so wish.

Psalm also has a few features to make it perform as well as possible on large codebases:

Multi-threaded mode
Using the --threads=[X] command line option will run Psalm's analysis stage on [X] threads. Useful for large codebases, it has a massive impact on performance.
Incremental checks
When using the --diff command line option, Psalm will only analyse files that have changed and files that reference them.

Example output

// somefile.php
<?php
$a = ['foo', 'bar'];
echo implode($a, ' ');

> ./vendor/bin/psalm somefile.php
ERROR: InvalidArgument - somefile.php:3:14 - Argument 1 of implode expects `string`, `array` provided

There are two main inspirations for Psalm:

Etsy's Phan, which uses nikic's php-ast extension to create an abstract syntax tree
Facebook's Hack, a PHP-like language that supports many advanced typing features natively, so docblocks aren't necessary.