Use output in a workflow step

action.php

<?= sprintf('::set-output name=var::%s', json_encode([
    [
        'id' => 'bar',
        'name' => 'foo',
    ],
    [
        'id' => 'bar2',
        'name' => 'foo2',
    ],
]));

main.yml

name: Use jq on output from step
on: [push]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v1
      - name: Run a command that sets an output with name var
        id: do-stuff # This is referred to below when fetching output
        run: php action.php
      - name: Use jq to process JSON output from above command
        run: |
          echo '${{ steps.do-stuff.outputs.var }}' | jq '{id: .[].id}'

actions.php should look like the below now I guess?:

<?= sprintf('name=var >> $GITHUB_OUTPUT%s', json_encode([ [ 'id' => 'bar', 'name' => 'foo', ], [ 'id' => 'bar2', 'name' => 'foo2', ], ]));