You probably already have the ODBC drivers installed; I obtained the latest by installing the Pervasive Client from http://www.pervasive.com/database/Home/Products/PSQLv12.aspx (current version is 12).
Then depending on 32 or 64 bit use the following connection string:
32bit:
$connect=odbc_connect("Driver={Pervasive ODBC Client Interface};ServerName=localhost;ServerDSN=DSNname;",
"Username", "Password", SQL_CUR_USE_ODBC);
64bit:
$connect=odbc_connect("Driver={Pervasive ODBC Interface};ServerName=localhost;ServerDSN=DSNname;",
"Username", "Password", SQL_CUR_USE_ODBC);
(You can check these strings with the ODBC data source administrator on Windows, use equivalent on other operating systems).