If assertExtension() fails, PEAR::DB will return DB_ERROR_EXTENSION_NOT_FOUND. But in your example, PEAR::DB returned DB_ERROR_NOSUCHDB. This means that the connection to the server could be established, but that the database was not found.

The reason for this is that the DSN, you're using, contains a database name that does not exist.

First: About error code and message
===================================

I add the following code to the first line of method 'DB_ifx::connect'.
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return $this->raiseError(DB_ERROR_EXTENSION_NOT_FOUND);
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This means immediately return method DB_ifx::connect with error DB_ERROR_EXTENSION_NOT_FOUND.
And display test.php again.  Output of test.php is ...
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test.php:40 => DB Error: no such database
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So something goes wrong here.
Because error code and message don't match. The message should be "extension not found".

To investigate more about this issue, I add the code "print_r($db)" to show the contents of error object.  Output is ...
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db_error Object ( [error_message_prefix] => [mode] => 1 [level] => 1024 [code] => -25 [message] => DB Error: no such database [userinfo] => ifx://test:test@test/test [callback] => )
test.php:41 => DB Error: no such database
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Developer can clearly see that code is -25 but message is "DB Error: no such database".

Should I submit another Bug report about this issue ?


Second: About if statement in DB_ifx::connect
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Thiks about following expression.
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$a || $b
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Meaning of this expression is "TRUE if either $a or $b is TRUE".
Ok ?

So following if statement appeard in DB_ifx::connect.
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if ((!DB::assertExtension('informix')) ||
(!DB::assertExtension('Informix'))) {
  return $this->raiseError(DB_ERROR_EXTENSION_NOT_FOUND);
}
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If system have two extension 'informix' and 'Informix', don't return with error.  But if system has only 'informix' or 'Informix' extension (not both), return with error.  Because one of DB::assertExtension() returns false and logical not change it's value to true.

What is the developer intention for above if statement ?
To use informix, system should have both 'informix' and 'Informix' extension ?

My system have only one extension 'Informix'.  So above condition is not met and return with error DB_ERROR_EXTENSION_NOT_FOUND. 
Is this a intended behavior ?

You are right about the second issue. A fix for this has been commited to CVS and will appear in the next release of PEAR::DB.

I suggest you file a new bug report for the first issue, so we can keep track of it more easily.