([0] => 'string1' [1] => 'string2' [2] => 'string3' )

('string1' => $lenght  'string2' => $lenght 'string3' => $lenght )

$arr = array('string1','string2','string3');
$length = "Your Val";
$newArr = array();
foreach($arr as $val){
    $newArr[$val] = $length;
}

print '<pre>';
print_r($newArr);
print '</pre>';

$a = array('Hello', 'World', 'PHP');
$a = array_flip($a);
array_walk($a, function(&$val, $key) {
    $val = strlen($key);
});
var_dump($a);