unset($res[$what_mas][$key]);

<?php

$mas = array ('a' => 4,

 'b' => 8,

 'c' => 1,

 'd' => 3,

 'e' => 9,

 'f' => 10

);

$mas = array (1,7,9,2,6,1,9);

$step = 0;

foreach ($mas as $k => $v)

{

if(($step % 2) === 0)

{

$res[0][$k] = $v;

}

else

{

$res[1][$k] = $v;  

}

$step++;

}

echo "<pre>";

print_r($mas);

echo "</pre>";

echo "<pre>";

print_r($res); 

echo "</pre>";

$steps = count($mas)+1;

$what_mas = 1; 

for($i=0; $i < $steps;  $i++)

{

foreach ($res[$what_mas] as $key => $val) 

{

$total_result[$key] = $val;

unset($res[$what_mas][$key]);

break;

}

$what_mas = !$what_mas;

}

echo "<pre>";

print_r($total_result);

echo "</pre>";

?>