i did a bit of google searching and saw PHPStan (which was mentinoed by @benanamen ) and decided to give it a try. You can install it with composer:

composer require --dev phpstan/phpstan

If you just run it, it will not detect undeclared properties, but if you add the -l flag with a level of 2 or higher, it will report 'undefined properties'. I created these files in a subdir, foo:

foo.php

<?php
class FooClass {
    // blah blah blah
}

file.php

<?php
require_once 'foo.php';
$foo = new FooClass();
$foo->some_new_property = 42;

and ran it:

$ vendor/bin/phpstan analyse -l 2 foo
 3/3 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ --------------------------------------------------------------- 
  Line   file.php                                                       
 ------ --------------------------------------------------------------- 
  4      Access to an undefined property FooClass::$some_new_property.  
 ------ --------------------------------------------------------------- 

 [ERROR] Found 1 error

This looks quite helpful. If anyone has experience with PHPStan or other static analysis tools, I'd be curious to hear any war stories.

EDIT: I got to wondering if PHPStan was smart enough to deal with instantiation of classes via factory methods. I seem to recall seeing this pattern a lot. To be extra sneaky I have my factory method return an instance of self rather than referring to the class name. Looks like PHPStan isn't smart enough to recogize the undeclared property assignment.
file2.php

<?php
require_once 'foo2.php';
$foo = FooClass2::factory();
$foo->some_new_property = 42;
var_dump($foo);

foo2.php

<?php
require_once 'foo.php';
class FooClass2 extends FooClass {
    // let's see if phpstan can detect factory methods
    public static function factory(){
        return new self();
    }
}

The analysis doesn't catch the undeclared property assignment in file2.php. If you crank up the reporting level to 6, it does complain that you haven't declared a return type for the factory method in foo2.php:

$ vendor/bin/phpstan analyse -l 6 foo
 4/4 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ --------------------------------------------------------------- 
  Line   file.php                                                       
 ------ --------------------------------------------------------------- 
  4      Access to an undefined property FooClass::$some_new_property.  
 ------ --------------------------------------------------------------- 

 ------ ----------------------------------------------------------- 
  Line   foo2.php                                                   
 ------ ----------------------------------------------------------- 
  5      Method FooClass2::factory() has no return type specified.  
 ------ ----------------------------------------------------------- 

 [ERROR] Found 2 errors