Why is Unity.exe crashing while running -batchmode?

I'm trying to set up a build farm to produce builds of my project. The setup is I have a perforce server set up on a windows server 2012 box. I've then got an IIS webserver running on it with PHP enabled.

Within PHP i am using exec(...) to execute console commands to trigger perforce to sync to head etc.

My next stage is to call exec("unity.exe" ...) to trigger unity to open in batch mode, build the project and exit. Triggering the exact command and parameters from a console window correctly produces the exe but running from the PHP causes the following log to be created.

Does anyone know why unity is crashing and what i can do to stop it?

BATCHMODE ARGUMENTS: C:/Program Files (x86)/Unity/Editor/unity.exe -batchmode -quit -logFile C:/Autobuild/11/logPc.txt -projectPath C:/Dev/projects/rts -buildWindows64Player C:/Autobuild/11/Pc/rts.exe Successfully changed project path to: C:/Dev/projects/rts

.... (shortening output) ....

Crash!!!

.... (shortening output) ....

========== OUTPUTING STACK TRACE ==================

(0x006DDF01) c:\buildagent\work\d63dfc6385190b60\runtime\scripting\backend\mono\scriptingbackendapi_mono.cpp (284 + 0x5): scripting_stack_trace_info_for + 0x4a1 (0x006DC0B1) c:\buildagent\work\d63dfc6385190b60\runtime\scripting\scripting.cpp (612 + 0x12): Scripting::LogException + 0x51 (0x0084AD56) c:\buildagent\work\d63dfc6385190b60\runtime\scripting\backend\scriptinginvocation.cpp (142 + 0x1c): ScriptingInvocation::Invoke + 0xa6 (0x0084B1FE) c:\buildagent\work\d63dfc6385190b60\runtime\scripting\backend\scriptinginvocation.cpp (114 + 0x0): ScriptingInvocation::Invoke + 0xe (0x0084AC05) c:\buildagent\work\d63dfc6385190b60\runtime\scripting\backend\scriptinginvocation.cpp (109 + 0x0): ScriptingInvocation::Invoke + 0x15 (0x00843F24) c:\buildagent\work\d63dfc6385190b60\runtime\mono\monomanager.cpp (1098 + 0x0): MonoManager::LoadAssemblies + 0x334 (0x008459E8) c:\buildagent\work\d63dfc6385190b60\runtime\mono\monomanager.cpp (1312 + 0x52): MonoManager::BeginReloadAssembly + 0x538 (0x00846041) c:\buildagent\work\d63dfc6385190b60\runtime\mono\monomanager.cpp (1136 + 0x0): MonoManager::ReloadAssembly + 0x21 (0x00D1BCA7) c:\buildagent\work\d63dfc6385190b60\editor\src\assetpipeline\monocompilationpipeline.cpp (1560 + 0x0): LoadDomainAndAssemblies + 0x77 (0x00D23AAC) c:\buildagent\work\d63dfc6385190b60\editor\src\assetpipeline\monocompilationpipeline.cpp (1589 + 0x0): LoadMonoAssembliesOrRecompile + 0xac (0x00FEA380) c:\buildagent\work\d63dfc6385190b60\editor\src\application.cpp (556 + 0x0): Application::InitializeProject + 0x1820 (0x0108D004) c:\buildagent\work\d63dfc6385190b60\editor\platform\windows\wineditormain.cpp (784 + 0x0): WinMain + 0xd04 (0x011A9FD4) f:\dd\vctools\crt_bld\self_x86\crt\src\crt0.c (275 + 0x1c): __tmainCRTStartup + 0x11a (0x7531919F) (KERNEL32): (filename not available): BaseThreadInitThunk + 0xe (0x77520BBB) (ntdll): (filename not available): RtlInitializeExceptionChain + 0x84 (0x77520B91) (ntdll): (filename not available): RtlInitializeExceptionChain + 0x5a

========== END OF STACKTRACE ===========