Description:
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Current implementation of Sqlite PDO does support ":memory:", but lacks support of "file:" scheme (URI filenames). This may be essential if one wants to change VFS. Trying to use URI filenames leads to "unable to open database" error. More information on URI filename can be found on official sqlite site: https://www.sqlite.org/c3ref/open.html#urifilenameexamples

Test script:
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$pdo = new PDO("sqlite:file:/opt/databases/mydb.sq2?vfs=unix-ecl");