Description:
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To make it extremely short: I believe a line within the _mssql_fetch_batch() function (in ext/mssql/php_mssql.c) should be changed (or the documentation).

Original:

if ( i < result->batchsize || result->batchsize == 0 ) 

Patched:

if ( i < result->batchsize || result->batchsize <= 0 ) 


In the original version, if 'i' is less than 'result->batchsize' OR 'result->batchsize' is equal to '0', then the next record is copied to the buffer and 'i' is incremented. This means, following the documentation and the most likely intention of the author (and the IF-clause), the 'batchsize' parameter of (for example) the 'mssql_query()' function is either a positive integer greater than zero, specifying the number of records to be fetched and stored within the buffer OR ANY other value, meaning that all records are fetched and stored within the buffer. 
But the source code only checks the latter case by comparing 'batchsize' to '0', where as it should perform the comparison with '<= 0', to include negative numbers, which can be supplied by the caller (there is no check performed anywhere against this case!). Otherwise, by supplying a negative value, the loop is left immediately in the first run, which might not be the intention of the author.

Thing is, I stumbled upon this oddity by inspecting the source code and cannot test it, since I have neither a Windows PC nor a MSSQL server to connect to, so this should be verified by someone, who does. Though I expect no problems at all, when patching this line AND modify slightly the documentation, that 0 or any negative value for 'batchsize' leads to fetching all records..