uniqueKeysOfList.php

<?php

$list = array(array("key1"=>"value","key4"=>"value"),array("key3"=>"value"),array("key2"=>"value"));

function uniqueKeysOfList($list) {
    $keys = [];

    foreach ($list as $obj) {
        foreach ($obj as $k => $v) {
            $keys[$k] = 1;
        }
    }
    
    return array_keys($keys);
}

var_export(uniqueKeysOfList($list));

?>