And the problem is..? The observed behaviour is correct. strcasecmp() returns 0, if the parameters match.

no, it isn't !!!

strcasecmp("B","AAA") should be equal to strcmp("B","AAA") 
at least in the results sign

the code for zend_binary_strcasecmp() 
in Zend/zend_operators.c is broken, the following diff
should fix it:

--- zend_operators.c    2000/03/01 13:59:51     1.53
+++ zend_operators.c    2000/03/05 19:56:43
@@ -1256,20 +1256,19 @@
 ZEND_API int zend_binary_strcasecmp(char *s1, uint len1, char *s2, uint len2)
 {
        unsigned char c1 = 0, c2 = 0;
+       int len;
 
-       if (len1 != len2 || !len1) {
-               return len1 - len2;
-       }
+       len = MIN(len1,len2);
 
-       while (len1--) {
+       while (len--) {
                c1 = tolower(*s1++);
                c2 = tolower(*s2++);
                if (c1 != c2) {
-                       break;
+                       return c1 - c2;
                }
        }
 
-       return c1 - c2;
+       return len1 - len2;
 }                                                                                  

php3_binary_strcasecmp was erroneously comparing string
size before content.  i commited the fix to php3.
zend still needs to be patched.

Thanks for the patch, I've applied it.