Prepared statements and unknown types

Could someone explain why the following doesn't work?

test=# PREPARE meow(unknown) AS
test-# SELECT $1 as meow;
ERROR:  could not determine data type of parameter $1

The problem is that using PDO in PHP, prepared statements aren't
possible if values are used instead of columns in the select list.

This appears to be allowed for MySQL and SQL Server.

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Thom Brown
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