Reproducing the problem:
------------------------
odbc_do($cid, 'Select * from table where datefield=DATE("16/02/2001")');

produces:
Warning: SQL error: [unixODBC][OpenLink][ODBC][Driver]Option Value Changed, SQL state [????
 in SQLExecDirect in
/www/data/datetest.php on line 12

Entering the query from the OpenLink odbctest program gives
the desired result.

Using DATE(02,16,2001) gives the same result. 

I've tried with php configured to use both iodbc and
unixODBC. Both produce the same result.

Queries not involving DATE() are working on the same setup. 

My configuration:
-----------------
'./configure' 
'--with-apxs=/usr/local/sbin/apxs' 
'--enable-versioning'
'--with-system-regex' 
'--with-config-file-path=/usr/local/etc'
'--disable-debug'
'--enable-track-vars'
'--without-gd'
'--disable-pear' 
'--without-mysql'
'--with-zlib'
'--with-mysql=/usr/local' 
'--with-unixODBC'
'--with-openlink'
'--with-magic-quotes'
'--enable-trans-sid' 
'--prefix=/usr/local'
'i386--freebsd4.2'

I'm running apache 1.3.14