Can we support syntax highlighting for PHP language?

In PHP, it is possible to embed a string literal using heredoc.

```
$query = <<< 'GQL'
query {
  foo {
     bar
  }
}
GQL;
```

The apostrophe can also be omitted.

```
$query = <<< GQL
query {
  foo {
     bar
  }
}
GQL;
```

How can we support syntax highlighting using this format?