Is there a way to get the raw SQL string executed when calling PDOStatement::execute() on a prepared statement? For debugging purposes this would be extremely useful.
I assume you mean that you want the final SQL query, with parameter values interpolated into it. I understand that this would be useful for debugging, but it is not the way prepared statements work. Parameters are not combined with a prepared statement on the client-side, so PDO should never have access to the query string combined with its parameters.
The SQL statement is sent to the database server when you do prepare(), and the parameters are sent separately when you do execute(). MySQL’s general query log does show the final SQL with values interpolated after you execute(). Below is an excerpt from my general query log. I ran the queries from the mysql CLI, not from PDO, but the principle is the same.
081016 16:51:28 2 Query prepare s1 from 'select * from foo where i = ?' 2 Prepare  select * from foo where i = ? 081016 16:51:39 2 Query set @a =1 081016 16:51:47 2 Query execute s1 using @a 2 Execute  select * from foo where i = 1
You can also get what you want if you set the PDO attribute PDO::ATTR_EMULATE_PREPARES. In this mode, PDO interpolate parameters into the SQL query and sends the whole query when you execute(). This is not a true prepared query. You will circumvent the benefits of prepared queries by interpolating variables into the SQL string before execute().
Re comment from @afilina:
No, the textual SQL query is not combined with the parameters during execution. So there’s nothing for PDO to show you.
Internally, if you use PDO::ATTR_EMULATE_PREPARES, PDO makes a copy of the SQL query and interpolates parameter values into it before doing the prepare and execute. But PDO does not expose this modified SQL query.
The PDOStatement object has a property $queryString, but this is set only in the constructor for the PDOStatement, and it’s not updated when the query is rewritten with parameters.
It would be a reasonable feature request for PDO to ask them to expose the rewritten query. But even that wouldn’t give you the “complete” query unless you use PDO::ATTR_EMULATE_PREPARES.
This is why I show the workaround above of using the MySQL server’s general query log, because in this case even a prepared query with parameter placeholders is rewritten on the server, with parameter values backfilled into the query string. But this is only done during logging, not during query execution.