Сравнить ключи массивов

$tariff_month_periods = array(
    0  => GetMessage('none'),
    1  => GetMessage('1_month'),
    2  => GetMessage('2_month'),
    3  => GetMessage('3_month'),
    6  => GetMessage('6_month'),
    9  => GetMessage('9_month'),
    12 => GetMessage('1_year'),
    24 => GetMessage('2_year'),
);

(
    [0] => 799.00
    [1] => 799.00
    [2] => 799.00
    [3] => 799.00
    [4] => 799.00
    [5] => 799.00
    [6] => 799.00
    [7] => 799.00
    [8] => 799.00
    [9] => 710.23
    [10] => 710.23
    [11] => 710.23
    [12] => 665.84
    [13] => 665.84
    [14] => 665.84
    [15] => 665.84
    [16] => 665.84
    [17] => 665.84
    [18] => 621.46
    [19] => 621.46
    [20] => 621.46
    [21] => 621.46
    [22] => 621.46
    [23] => 621.46
    [24] => 599.25
)

$arr1 = [......];
$arr2 = [......];
$result = array_diff_key($arr1, $arr2);
if(is_array($result))
    {
        foreach($result as $item)
            {
                $arr3[] = $item;
                break;
            }
    }

$ar_periods=[];
foreach($price as $key => $value){
    if((isset($tariff_month_periods[$key]) && ($value != 0))){
        $ar_periods[] = $tariff_month_periods[$key];
    }
}

$value != 0

$value != NULL

if((isset($tariff_month_periods[$key],$value))

isset($tariff_month_periods[$key] && isset($value)

$arr1 = [......];
$arr2 = [......];
$result = array_diff_key($arr1, $arr2);
if(is_array($result))
    {
        foreach($result as $item)
            {
                $arr3[] = $item;
                break;
            }
    }

isset($tariff_month_periods[$key]) && isset($value)

<?php
$array1 = array('blue'  => 1, 'red'  => 2,'purple' => 4);
$array2 = array('green' => 5, 'blue' => 6, 'yellow' => 7, 'cyan'   => 8);
$arr = array_intersect_key($array1, $array2);
    print_r($arr);
    ?>

<?php
$array1 = array('blue'  => 1, 'red'  => 2,'purple' => 4,"");
$array2 = array('green' => 5, 'blue' => 6, 'yellow' => 7);
         $a = array_filter($array1);//убераем  " " 
         $b = array_filter($array2);//убераем " " 
$arr[] = array_intersect_key($a, $b); //схождение ключей
$arr[] = array_diff_key($a,$b); // расхождение ключей
print_r($arr);
    ?>

(
    [0] => 799.00
    [1] => 799.00
    [2] => 799.00
    [3] => 799.00
    [4] => 799.00
    [5] => 799.00
    [6] => 799.00
    [7] => 799.00
    [8] => 799.00
    [9] => 710.23
    [10] => 710.23
    [11] => 710.23
    [12] => 665.84
    [13] => 665.84
    [14] => 665.84
    [15] => 665.84
    [16] => 665.84
    [17] => 665.84
    [18] => 621.46
    [19] => 621.46
    [20] => 621.46
    [21] => 621.46
    [22] => 621.46
    [23] => 621.46
    [24] => 599.25
)

Array
(
    [0] => 0
    [1] => 0
    [2] => 0
    [3] => 0
    [4] => 0
    [5] => 0
    [6] => 0
    [7] => 0
    [8] => 0
    [9] => 0
    [10] => 0
    [11] => 0
    [12] => 1249.92
    [13] => 1249.92
    [14] => 1249.92
    [15] => 1249.92
    [16] => 1249.92
    [17] => 1249.92
    [18] => 1249.92
    [19] => 1249.92
    [20] => 1249.92
    [21] => 1249.92
    [22] => 1249.92
    [23] => 1249.92
    [24] => 1124.93
)

var_dummp(array_intersect_assoc($arr1, $arr2));

for($i = 0; $i <= 25; $i++){
    if($price[$i++] != $price[$i]){
       $ar_periods[$i++] = $price[$i++];
    }
}

for($i = 0; $i <= 24; $i++){
    if($price[$i] != $price[$i++]){
       $ar_periods[$i] = $price[$i];
    }
}

Array
(
    [0] => 0
    [1] => 0
    [2] => 0
    [3] => 0
    [4] => 0
    [5] => 0
    [6] => 0
    [7] => 0
    [8] => 0
    [9] => 0
    [10] => 0
    [11] => 0
    [12] => 1249.92
    [13] => 1249.92
    [14] => 1249.92
    [15] => 1249.92
    [16] => 1249.92
    [17] => 1249.92
    [18] => 1249.92
    [19] => 1249.92
    [20] => 1249.92
    [21] => 1249.92
    [22] => 1249.92
    [23] => 1249.92
    [24] => 1124.93
)

for($i = 0; $i < 24; $i++){
    if($price[$i++] !== $price[$i]){
       $ar_periods[$i] = $price[$i];
    }
}

for($i = 0; $i < 24; $i++){
    if($price[++$i] !== $price[$i]){
       $ar_periods[$i] = $price[$i];
    }
}

for($i = 0; $i < 24; $i++){
    if($price[$i++] !== $price[$i]){
       $ar_periods[$i] = $price[$i];
    }
}

for($i = 0, $j = 0; $i < 24; $i++){
    if($price[$j] !== $price[$i]){
       $ar_periods[$i] = $price[$i];
             $j++;
    }
}