PHP: Date range overlap check

<?php // Function to test for time overlap 
// $start_time    A start date YYYY-MM-DD HH:MM:SS 
// $end_time      An end date YYYY-MM-DD HH:MM:SS 
// $times         An array of times to match against 
// Returns true if there is an overlap false if no overlap is found function time_overlap($start_time, $end_time, $times){ 
    $ustart = strtotime($start_time); 
    $uend   = strtotime($end_time); 
    foreach($times as $time){ 
        $start = strtotime($time["start"]); 
        $end   = strtotime($time["end"]); 
        if($ustart <= $end && $uend >= $start){ 
            return true; 
        } 
    } 
    return false; 
} // A test list of times $list_of_times = array( 
    array( 
        "start" => "2012-01-01 00:00:00", 
        "end" => "2012-01-30 00:00:00" 
    ), 
    array( 
        "start" => "2012-02-01 00:00:00", 
        "end" => "2012-02-30 00:00:00" 
    ), 
    array( 
        "start" => "2012-03-01 00:00:00", 
        "end" => "2012-03-30 00:00:00" 
    ) 
); // Test some times if(!time_overlap("2012-03-15 00:00:00", "2012-04-01 00:00:00", $list_of_times)){ 
    echo "No overlap found adding to array!<br />"; 
    $list_of_times[]["start"] = "2012-03-15 00:00:00"; 
    $list_of_times[]["end"] = "2012-04-01 00:00:00"; 
}else{ 
    echo "Overlap found time not added to array!<br />"; 
} 

if(!time_overlap("2012-04-15 00:00:00", "2012-05-01 00:00:00", $list_of_times)){ 
    echo "No overlap found adding to array!<br />"; 
    $list_of_times[]["start"] = "2012-03-15 00:00:00"; 
    $list_of_times[]["end"] = "2012-04-01 00:00:00"; 
}else{ 
    echo "Overlap found time not added to array!<br />"; 
}