bpws:getVariableData('input','payload', '/global:PhilosopherRequest/global:iterations')

Section 14.1 of the BPEL4WS spec WS-BPEL1.1 explains….

Expressions

These extensions refer to the Expressions feature of BPEL4WS.

The first extension defines a standard fault for erroneous use of the XPath 1.0 function defined for extracting global property values from variables.

bpws:getVariableProperty ('variableName', 'propertyName')

The first argument names the source variable for the data and the second is the qualified name (QName) of the global property to select from that variable (see Message Properties). If the given property does not appear in any of the parts of the variable's message type or the given property definition selects a node set of a size other than one, then the standard fault bpws:selectionFailure MUST be thrown by a compliant implementation.

The second extension defines an additional XPath 1.0 function usable only in executable processes. This function extracts arbitrary values from variables.

bpws:getVariableData ('variableName', 'partName', 'locationPath'?)

The first argument names the source variable for the data, the second names the part to select from that variable, and the third optional argument, when present, provides an absolute location path (with '/' meaning the root of the document fragment representing the entire part) to identify the root of a subtree within the document fragment representing the part. The return value of this function is a node set containing the single node representing either an entire part (if the third argument is absent) or the result of the selection based on the locationPath. If the given locationPath selects a node set of a size other than one during execution, then the standard fault bpws:selectionFailure MUST be thrown by a compliant implementation.

I am working on a patch and will roll out asap.