gistfile1.txt

Array
(
    [0] => Array
        (
            [event_id] => 247093
            [date_schedule] => 2014-05-14T02:30:00
            [city] => San Diego
            [venue] => Humphreys Concerts by the Bay
            [buy_link] => http://www.axs.com/events/247093/lindsey-stirling-with-dia-frampton-tickets
        ),
    [1] => Array
        (
            [event_id] => 247094
            [date_schedule] => 2014-05-14T02:30:00
            [city] => San Diego
            [venue] => Humphreys Concerts by the Bay
            [buy_link] => http://www.axs.com/events/247093/lindsey-stirling-with-dia-frampton-tickets
        ),
    [2] => Array
        (
            [event_id] => 247095
            [date_schedule] => 2014-05-14T02:30:00
            [city] => San Diego
            [venue] => Humphreys Concerts by the Bay
            [buy_link] => http://www.axs.com/events/247093/lindsey-stirling-with-dia-frampton-tickets
        )

)

// to

Array
(
    [247093] => Array
        (
            [event_id] => 247093
            [date_schedule] => 2014-05-14T02:30:00
            [city] => San Diego
            [venue] => Humphreys Concerts by the Bay
            [buy_link] => http://www.axs.com/events/247093/lindsey-stirling-with-dia-frampton-tickets
        )
    [247094] => Array
        (
            [event_id] => 247093
            [date_schedule] => 2014-05-14T02:30:00
            [city] => San Diego
            [venue] => Humphreys Concerts by the Bay
            [buy_link] => http://www.axs.com/events/247093/lindsey-stirling-with-dia-frampton-tickets
        )
    [247095] => Array
        (
            [event_id] => 247093
            [date_schedule] => 2014-05-14T02:30:00
            [city] => San Diego
            [venue] => Humphreys Concerts by the Bay
            [buy_link] => http://www.axs.com/events/247093/lindsey-stirling-with-dia-frampton-tickets
        )
)

<?php 
$newArray = array();

for($i= 0 , $i <= count($array), $i++){
$newArray[$array[$i]['event_id']] = $array[$i];
}