Out of the box, VSCode seems to not encounter this particular issue with SQL highlighting within a PHP string. The highlighting seems to interpret the PHP variable as a string, when it probably should fallback to the language of the file / outer scope and check what sort of class it should have - in this particular case, it should be interpreted as a PHP variable.
Also, I might create another issue for the following, but not sure how easy it is to check for this problem:
<?php
$editors = '"'.implode('","', ['vim', 'atom', 'vscode']).'"';
$results = database::select_one_column(
"SELECT user.id
FROM user
WHERE user.healthy = true
AND user.editor IN ($editors)"
);
$editors is styled like any other plain PHP string.
It is also worth noting that disabling this package, will correctly display PHP string interpolation.