From the docs:
This means that the expression you want to be matched is a single, text (string) value. If you're using a prepared statement, in PHP, with PDO, that means a single question mark (?) or a single ":name" placeholder if you're going for named parameters.
A quick demo example:
<?php
$db = new PDO("sqlite::memory:");
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$db->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
$db->exec("CREATE VIRTUAL TABLE example USING FTS5 (a, b)");
$db->exec("INSERT INTO example (a, b) VALUES (