How do I send an array of strings to PG from PHP without the former becoming confused?
Use instead:
SELECT * FROM table WHERE label &lt;&gt; ALL ($1);

Then you can pass an actual array for $1 - while (NOT) IN expects a list of values. See:

How to use ANY instead of IN in a WHERE clause with Rails?

If NULL values can be involved on either side, either query is problematic. You may want the form:
SELECT * FROM table WHERE (label = ANY ($1) IS NOT TRUE);

See:

Why does my SELECT query not return null values?