<?php

$dados = array
(
    0 => 'MemTotal:        4060964 kB',
    1 => 'MemFree:         3630320 kB',
    2 => 'MemAvailable:    3789472 kB',
    3 => 'Buffers:           93040 kB',
    4 => 'SwapCached:            0',
    6 => 'Active:           306312 kB'
);


$dadosNew = array();
foreach($dados as $key => $value)
{
	$valueNew = explode(":", $value);
	$dadosNew[$valueNew[0]] = trim($valueNew[1]);
}

var_dump($dadosNew);