<?php

//METHOD 1
$string1 = 'JOHN';
$string2 = 'SMITH';

$count_str1 = strlen($string1);
$count_str2 = strlen($string2);

if($count_str1 > $count_str2){
    merge_strings($string1,$string2);
}else{
    merge_strings($string2,$string1);
}

function merge_strings($string2,$string1){
$new_str = '';
    $arr = str_split($string1);

    for($i=0;$i<=count($arr);$i++){
        if(!empty($string1[$i]))
        $new_str .= $string1[$i];
        if(!empty($string2[$i]))
        $new_str .= $string2[$i];
    }

    echo $new_str;
}