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 type. mixed types can sometimes mask bugs, so keeping track of them helps you avoid a number of common pitfalls.

  • Intelligent 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.

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

  • Multi-threaded mode
    Wherever possible Psalm will run its analysis in parallel to save time. 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

Given a file implode_strings.php:

$a = ['foo', 'bar'];
echo implode($a, ' ');
> ./vendor/bin/psalm implode_strings.php
ERROR: InvalidArgument - somefile.php:3:14 - Argument 1 of implode expects `string`, `array` provided (see https://psalm.dev/004)


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.