使用嵌套循环构建具有唯一值的数组（PHP / XML）

Question

I'm using PHP to collect attributes from an XML file.

Eg:

<?xml version="1.0"?> 
    <menu restaurant="edible things"> <!-- Root Node -->
        <section category="Fruit">
            <item>  Apple
                <price variant="small"> 2.50    </price>    
                <price variant="large"> 3.75    </price>
            </item>
            <item>  Banana  
                <price variant="small"> 1.85    </price>    
                <price variant="medium">    2.00    </price>    
                <price variant="large"> 2.85    </price>
            </item>
        </section>
        <section category="Instruments">            
            <item>  Piano   
                <price variant="red">   100.00  </price>    
                <price variant="yellow">    3000.00 </price>
            </item>
            <item>  Tuba    
                <price variant="red">   60.00   </price>    
                <price variant="yellow">    250.00  </price>
            </item>
        </section>      
    </menu>

My goal is to create an array/list of the price variants with no duplicates. To that end I have implemented the following PHP (collected in a web page):

<?php 
foreach ($xml->section as $section) {
    echo '<h2>'.$section['category'].'</h2><br />';
    $arr = array();
    foreach ($section->item as $item) {
        foreach ($item->price as $price) {
            if (!in_array($price['variant'], $arr)) {
                array_push($arr, $price['variant']);
            }
            else {
            }
        }
    }
    $arrlength = count($arr);
    for($x = 0; $x < $arrlength; $x++) {
        echo $arr[$x];    
        echo '<br>';
        }
}
?>

Unfortunately, in this instance I get an array that contains every attribute value, including duplications. If I cut either foreach loop out, I get only (a) attributes for the first item in section, or (b) the first variant for all items, respectively.

I realize that I'm having a problem with understanding how nesting works. How do you build an array with only unique values?