Setup: RH Linux 6.0/Oracle EE 8.1.5/Apache 1.3.9/PHP 3.012
Script to reproduce problem:
---------(start code)
      $stmt = "insert into... values..."; //will raise a unique constraint violation error
      $stmt = ociparse($conn, $stmt);
      @ociexecute($stmt);
      $err1 = ocierror();
      $err2 = ocierror($stmt);
      $err3 = ocierror($conn);
      print "Error messages:<br>(1)".$err1."<br>(2)".$err2."<br>(3)".$err3."<br>";
      print "PHP error: ".$php_errormsg;
---------(end code)

This will output (when viewed in HTML browser):
---------(start output)
(1)
(2)Array
(3)
PHP error: OCIStmtExecute: ORA-00001: unique constraint (WEEZ.SYS_C00894) violated
---------(end output)

Notice that the ocierror() function is totally useless in this example. 

Other probably irrelevant info:
PHP cofigure line: configure --with-apxs=... --with-apache=... --with-oracle=... --with-xml

php.ini file: 
include_path=...
auto_prepend_file=...
track_vars = on
magic_quotes_gpc = on
sendmail_path /usr/sbin/sendmail -t
track_errors = on
short_open_tag = off