$Medio=''; //variable to handle the result

$MedioPago='12345'; //variable with numbers given

$chars = str_split($MedioPago); //split the numbers


foreach($chars as $char)
{
    if($char=='1')
    {
        $Medio.='"codigo": "01",'.PHP_EOL;
    }
    else if($char=='2')
    {
        $Medio.='"codigo": "02",'.PHP_EOL;
    }
    else if($char=='3')
    {
        $Medio.='"codigo": "03",'.PHP_EOL;
    }
    else if($char=='4')
    {
        $Medio.='"codigo": "04",'.PHP_EOL;
    }
    else if($char=='5')
    {
        $Medio.='"codigo": "05",'.PHP_EOL;
    }
}

$Medio = rtrim($Medio,',');