<?php

    $week = array( 'Monday' => 'mon', 'Tuesday' => 'tue', 'Wednesday' => 'wed', 
                   'Thursday' => 'thur', 'Friday' => 'fri', 'Saturday' => 'sat', 
                   'Sunday' => 'sun' );

    function shiftArray( &$a, $k ) {
        $k = array_search( $k, array_keys( $a ) );
        $a = array_merge( array_slice( $a, $k ), array_slice( $a, 0, $k ) );
    }
    
    shiftArray( $week, "Wednesday" );

    print_r( $week );

Array
(
    [Wednesday] => wed
    [Thursday] => thur
    [Friday] => fri
    [Saturday] => sat
    [Sunday] => sun
    [Monday] => mon
    [Tuesday] => tue
)