static const char * _php_db2_instance_name;
...

PHP_MINIT_FUNCTION(ibm_db2)
{
    /* Declare variables for DB2 instance settings */
    char * tmp_name;
    char * instance_name;
...

#ifndef PHP_WIN32
    tmp_name = INI_STR("ibm_db2.instance_name");
    if (NULL != tmp_name) {
        instance_name = (char *)malloc(strlen(DB2_VAR_INSTANCE) + strlen(tmp_name) + 1);
        strcpy(instance_name, DB2_VAR_INSTANCE);
        strcat(instance_name, tmp_name);
        putenv(instance_name);
        _php_db2_instance_name = instance_name;
    }
#endif

...

PHP_MSHUTDOWN_FUNCTION(ibm_db2)
{
    UNREGISTER_INI_ENTRIES();

    if ( IBM_DB2_G(henv) ) {
        SQLFreeHandle ( SQL_HANDLE_ENV, IBM_DB2_G(henv) );
    }
    if (NULL != _php_db2_instance_name) {
        free((char *)_php_db2_instance_name);
    }

    return SUCCESS;
}