xpath - get first 10 items of selected set

Have the position condition operate on the result nodeset of your initial condition:

//parent/child[sex='male'][position() >= 10 and position() < 21]

I want to select a sub set of nodes ( for pagination purposes ) from a set.

$nodes = $xml->query(//parent                         /child[sex = 'male'                                 and position() >= 10                                 and position() < 21]); 

If I'm not mistaken that would only select male children who are the 10th to 20th child.

what I need is to select the first 10-20 (or 30-40) males in the set...

You are mistaken...

//parent/child           [sex = 'male'          and            position() >= 10          and            position() < 21           ] 

Selects all child elements (of any parent element in the XML document) that have a sex child with sting value "male" and that are one of the 10th to 20th child children of their parent.

There could be only a few, or even none such elements.

What you want is:

1. Selects all child elements (of any parent element in the XML document) that have a sex child with sting value "male"

2. From the ones selected in step 1 above select only those in position 10 to 20

So, for step 1:

//parent/child[sex = 'male']

and adding step 2:

//parent/child[sex = 'male']                [position() >= 10               and                 not(position() > 20                ]