How to get a specific element in SimpleXML?

Question

PHP

there is this file:

<?xml version="1.0" encoding="utf-8" ?>
<bikes>
    <bike>
        <model>First</model>
        <speedNumber>4</speedNumber>
        <sizes>100</sizes>
        <amount>100</amount>
    </bike>
    <bike>
        <model>Second</model>
        <speedNumber>3</speedNumber>
        <sizes>300</sizes>
        <amount>150</amount>
    </bike>
    <bike>
        <model>Third</model>
        <speedNumber>4</speedNumber>
        <sizes>300</sizes>
        <amount>300</amount>
    </bike>
</bikes>

How to get the parent elements of exactly the 1st bike element?
I try like this:

$xml=simplexml_load_file("bike.xml") or die('Cannot load file');
foreach ($xml->children(0) as $bikes) {
    foreach ($bikes->children() as $childs){
        echo $childs->getName()."=".$childs ." ";
    }
    echo "<br>";
}

But that doesn't work. What am I doing wrong please tell me