here is an example from Symfony Documentation of Jobeet Model
The file is 020_jobs.yml

JobeetJob:
# Starts at the beginning of the line (no whitespace before)
<?php for ($i = 100; $i <= 130; $i++): ?>
  job_<?php echo $i ?>:
    category_id: programming
    company: Company <?php echo $i."\n" ?>
    position: Web Developer
    location: Paris, France
    description: Lorem ipsum dolor sit amet, consectetur adipisicing elit.
    how_to_apply: |
      Send your resume to lorem.ipsum [at] company_<?php echo $i ?>.sit
    is_public: true
    is_activated: true
    token: job_<?php echo $i."\n" ?>
    email: job@example.com
<?php endfor; ?>

As far as I see this is actually completely opposite task to that i thought it was initially – our example is actually PHP file but with YAML "outside".
The good news is that such cases are already supported in current version:
Just open settings and reassign yml extension to PHP file type and than select desired template language under "Template Data Languages"

Does this cause the YAML syntax outside the PHP tags to be syntax-colored/navigable/etc. as YAML? Or will it get treated as plaintext/HTML?

Yes.
Why not just try it?..

Good point. I tried it out, but it didn't work:

Everything inside the <?php ?> tags was correctly recognized as PHP, but there was no syntax coloring for the YAML code outside the <?php ?> tags, the Structure window was empty, etc.

@Phoenix Zerin
Syntax highlighting was OK for me. But Structure panel is not showing Yaml structure, only PHP variables in PHP tab.

Thanks for verifying. I missed the part about changing the template data language.

Does that have to be configured for every .yml file in the project, or can I blanket-apply the Template Data Language setting to every applicable file?

@Phoenix Zerin
Template Data Language can be applied to a specific file or folder (so ALL files in that folder will inherit the new value). You cannot mass-apply it using filename mask (e.g. *.yml).

Doesn't that seem a little cumbersome for end users? Instead of PhpStorm being aware of PHP syntax in YAML files by default, users have to tell PhpStorm to treat all YAML files as PHP files and then manually override the setting on a per-file or per-folder basis.

It also would necessitate storing the .idea metadata folder in the source control repository, or else each developer on the team would have to duplicate this configuration workaround in her own environment for each project.

.idea folder is supposed to be stored in vcs. (excluding the workspace.xml file)