PHP unified ODBC and PHP PDO ODBC do not support Unicode. Those PHP ODBC libraries are using ANSI ODBC APIs (SQLxxxA APIs) instead of the Unicode ODBC API (SQLxxxW APIs)"
More information available on https://www.progress.com/tutorials/odbc/unicode
The reason UTF-8 works with PHP when using Mysql (another type of database often used with PHP) is because MySQL does not use 'odbc_do' (unified ODBC PHP library) but for example 'mysqli_query' from a different PHP library (mysqli). There is also more than one PHP library available for connecting to MySQL, such a custom PHP extension does not exist for OpenEdge. (only the generic PHP unified ODBC and PHP PDO libraries can be used for OpenEdge ODBC connections from within PHP)