How to disable highlighting for SQL code in phpstorm?

SQL Language is automatically injected in strings that contain SQL code (which are detected by the typical patterns, e.g. select xx from etc). It also injected in HEREDOC/NOWDOC if you use special label (SQL).

You can disable these (selected on the screenshot below) and any other unwanted injection rules or create your own at Settings/Preferences | Editor | Language Injections.

P.S. Since you do not need any SQL/DB support at all, you may just disable SQL/database support plugins completely.

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If you do like such injections in general but just do not want them in a specific place only (e.g. because of the false positive match) then you can force plain text in that string. For example:

$str = /** @lang Text */ 'Select all entries from my cool database';

Please note that such comment must be placed just before the actual string (so it can be used in function call params or alike), not before the whole variable assignment/statement like ordinary PHPDoc comments.

P.S. The same is possible other way around: force SQL in some string that is not autodetected by Language Injection rules (e.g. when string is split into concatenated bits or uses unknown/unexpected sequence/syntax).