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how to separate array with commas?
<?php
    if ( is_array($_categories) ) {
        foreach ($_categories as $key => $value) {
            $out = array();
            array_push($out, $value);
            echo implode(', ', $out);
        }
    }
    else {
        echo '<li>There are no saved values yet.</li>';
    }
?>

echo implode(', ', $_categories);

$out = array();   //putting outside of the loop
foreach ($_categories as $key => $value) {
    array_push($out, $value);
}
echo implode(', ', $out);   //putting outside of the loop