Phing, create uniquely named build directory in a tmp directory task

gistfile1.xml

<target name="createbuilddir">
        <php expression="substr(md5(microtime(true)), 0, 1)" returnProperty="uniquestamp" />
        <property name="build.dir" value="${build.tmp}/phing_build_${uniquestamp}" override="true" />
        <echo msg="Testing tmp dir ${build.dir}" />
        <if>
            <available file="${build.dir}" type="dir" value="Yes"/>
            <then>
                <phingcall target="createbuilddir">
                    <property name="build.tmp" value="${build.tmp}" />
                </phingcall>
            </then>
        </if>
        <mkdir dir="${build.dir}" />
    </target>

<!-- this seems to work while a directory does not exists but errors


BuildPackage > init:

     [echo] Initializing properties

BuildPackage > createbuilddir:

      [php] Evaluating PHP expression: substr(md5(microtime(true)), 0, 1)
     [echo] Testing tmp dir /Users/bas/Documents/Projects/Lumasol/store.lumasol.nl/src/application/../data/tmp/phing_build_e
[phingcall] Calling Buildfile '/Users/bas/Documents/Projects/Lumasol/store.lumasol.nl/src/data/phing-build-files/buildPackage.xml' with target 'createbuilddir'
[phingcall] phing task calling its own parent target
       [if] Error in IfTask
Execution of target "createbuilddir" failed for the following reason: /Users/bas/Documents/Projects/Lumasol/store.lumasol.nl/src/data/phing-build-files/buildPackage.xml:60:12: /Users/bas/Documents/Projects/Lumasol/store.lumasol.nl/src/data/phing-build-files/buildPackage.xml:63:34: Execution of the target buildfile failed. Aborting.

BUILD FAILED
/Users/bas/Documents/Projects/Lumasol/store.lumasol.nl/src/data/phing-build-files/buildPackage.xml:60:12: /Users/bas/Documents/Projects/Lumasol/store.lumasol.nl/src/data/phing-build-files/buildPackage.xml:63:34: Execution of the target buildfile failed. Aborting.
Total time: 0.0741 seconds