Merge 2 JSON Files in PHP

[Editor]

I have two JSON data.

One of them contains only 2 values. The other one contains six values.
In line with the numbers in the first JSON, I want to separate the second JSON data.

For example:

3 -> val3
5->  val5

First JSON:

{"0":"3","1":"5"}

Second JSON:

{"val1":"ValueOne","val2":"ValueSecond","val3":"ValueThree","val4":"4","val5":"ValueFive","val6":"ValueSix"}

$first = json_decode($jsonFile1);
$second = json_decode($jsonFile2);

foreach ($first as $key => $firstvalue) {
  foreach ($second as $secondvalue) {
    if (substr($firstvalue, -1) == $secondvalue) {  <-- 'valX' => 1
       echo "<strong>". $firstvalue . "</strong><br>";
    } else { 
       echo "<em>". $firstvalue . "</em>";
    } 
  }
}

Result:

---

• ValueOne
• ValueOne
• ValueSecond
• ValueSecond
• ValueThree
• ValueThree
• ValueFour
• ValueFour
• ValueFive
• ValueFive
• ValueSix
• ValueSix

What could be the reason?

[Editor]

Nobody?