Starts debugging to {dump_directory}/apd_dump_{process_id}, if dump_directory is not set, then the apd.dumpdir setting from the php.ini file is used.
debug_level is an integer which is formed by adding together the following values:
FUNCTION_TRACE      1
        ARGS_TRACE          2
        ASSIGNMENT_TRACE    4
        STATEMENT_TRACE     8
        MEMORY_TRACE        16
        TIMING_TRACE        32
        SUMMARY_TRACE       64 | 
I would seriously not recommend using MEMORY_TRACE. It is very slow and does not appear to be accurate (great, huh?) also ASSIGNMENT_TRACE is not implemented. So, to turn on all functional traces (TIMING, FUNCTIONS, ARGS SUMMARY (like strace -c)) use the value 99