The lower-caseXPath function is not available, but you can use the translateXPath 1.0 function to convert text to lowercase, for example. for the English alphabet:
translate(text(),'ABCDEFGHIJKLMNOPQRSTUVWXYZ','abcdefghijklmnopqrstuvwxyz')
*=, contains , :
doc.search("//*[contains(translate(text(),'ABCDEFGHIJKLMNOPQRSTUVWXYZ','abcdefghijklmnopqrstuvwxyz'),'philip morris')]")