When compiling php, I ran into the problem of not having size_t defined in the file 'functions/formatted_print.c' . This source file included <math.h> which needs size_t to be defined. Since size_t is defined in <stdio.h> on this Irix system, I merely switched the order of the include files in 'formatted_print.c' so that stdio.h came before math.h. This fixed the problem and compilation finished without further incident.