PHP SimpleXML Optional Child Nodes

I'm trying to separate my child nodes according their children but I can't figure out how to validate whether the node has a value.

Here's my XML:

<?xml version="1.0"?>
<testcase>
 <steps>
 <step id="one">
    <command>gettitle</command>
 </step>
 <step id="two">
    <command>click</command>
    <parameter>id=searchByAddressSubmit</parameter>
 </step>
 </steps>
</testcase>

Here's my Code:

$testcase = new SimpleXMLElement($xml);
foreach($testcase->steps->step as $step) {
    echo $step->parameter;
    echo $step->command;    
    if(empty($step->parameter)) {
        echo $step>command;
    }
} 

The result should be:

gettitle

I've tried empty(), array_key_exists(), and is_null() but nothing seems to pick a missing value. Any ideas?