Unable to scan only modified files with php lint

Build Environment
1

Hi,
im trying to run an ant build which executes some php linting function only over modified files. this used to work using jenkins, but lost this possibility when using circleci.

<target name="lint">
    <apply executable="php" failonerror="true">
        <arg value="-l"/>
        <fileset dir="${basedir}">
            <include name="system/application/**/*.php"/>
            <modified />
        </fileset>
    </apply>
</target>

is there any way to run this command only on files modified in the current build? what might be happening? which is the circleci way of doing this?

thanks,
kenny