ループを使用する代わりに、いつでもを使用して文字列にフラット化しjson_encode()、文字列の置換を実行しjson_decode()てから、配列に戻すことができます。
function replaceKey($array, $old, $new)
{  
    //flatten the array into a JSON string
    $str = json_encode($array);
    // do a simple string replace.
    // variables are wrapped in quotes to ensure only exact match replacements
    // colon after the closing quote will ensure only keys are targeted 
    $str = str_replace('"'.$old.'":','"'.$new.'":',$str);
    // restore JSON string to array
    return json_decode($str, TRUE);       
}
これで、既存のキーとの競合がチェックされなくなり(文字列比較チェックを追加するのに十分簡単)、大規模な配列での単一の置換には最適なソリューションではない可能性があります。置換の文字列は、任意の深さの一致がすべて1回のパスで置換されるため、置換を効果的に再帰的にすることです。
$arr = array(
    array(
         'name'     => 'Steve'
        ,'city'     => 'Los Angeles'
        ,'state'    => 'CA'
        ,'country'  => 'USA'
        ,'mother'   => array(
             'name'     => 'Jessica'
            ,'city'     => 'San Diego'
            ,'state'    => 'CA'
            ,'country'  => 'USA'
        )
    )
    ,array(
         'name'     => 'Sara'
        ,'city'     => 'Seattle'
        ,'state'    => 'WA'
        ,'country'  => 'USA'
        ,'father'   =>  array(
             'name'     => 'Eric'
            ,'city'     => 'Atlanta'
            ,'state'    => 'GA'
            ,'country'  => 'USA'
            ,'mother'   => array(
                 'name'     => 'Sharon'
                ,'city'     => 'Portland'
                ,'state'    => 'OR'
                ,'country'  => 'USA'
            )
        )
    )
);
$replaced = replaceKey($arr,'city','town');
print_r($replaced);
出力
Array
(
    [0] => Array
        (
            [name] => Steve
            [town] => Los Angeles
            [state] => CA
            [country] => USA
            [mother] => Array
                (
                    [name] => Jessica
                    [town] => San Diego
                    [state] => CA
                    [country] => USA
                )
        )
    [1] => Array
        (
            [name] => Sara
            [town] => Seattle
            [state] => WA
            [country] => USA
            [father] => Array
                (
                    [name] => Eric
                    [town] => Atlanta
                    [state] => GA
                    [country] => USA
                    [mother] => Array
                        (
                            [name] => Sharon
                            [town] => Portland
                            [state] => OR
                            [country] => USA
                        )
                )
        )
)