<?php 
    $a = array(array('cf'=>'joan', 'nf'=>'5'), array('cf'=>'lol', 'nf'=>'55'));
    $new = array();

    foreach( $a as $k ) {
        $new[$k[cf]] = $k[nf];
    }
    print_r( $new );

Array
(
    [joan] => 5
    [lol] => 55
)