Juniper SRX3400 doesn't show sessions per module

Yes, I see issue, currently graph module collect only single node sessions.

Im working about improvements.

The code as it stands today, assumes that all the information is in the first entry. This is true for a branch SRX where there is only one FPC. However, with higher end SRX's (3000, 4000 and 5000) there are potentially multiple FPC's each with their own set of counters for session counts etc... Also, these FPC numbers quite often do not start at zero. The approach used seems like a fairly generic method and really a more specific mechanism is required to graph this data which will pull the session information for each FPC rather than just the first one. 

That's true, but:
1) Juniper referes to the same MIB for SRX3k and SRX5k series: https://www.juniper.net/techpubs/en_US/junos12.3x48/information-products/topic-collections/mibs-and-traps-ref/mibs-and-traps-ref/topic-45121.html#jd0e120325#aSSGoEb7BiKIwgVF.97
2) snmpwalk returns correct values
3) it's rather clear from provided dump that php code fails to map SPU slot index to actual SNMP values, and always assigns index 0. So it's actually working for low-end SRX with only 1 slot.

Your SRX3400 isn't an SRX5000.

Sorry, I meant indexes not matching.