Merge two strings

Merges two strings in a way that a pattern like ABABAB will be the result.

/**
 * Merges two strings in a way that a pattern like ABABAB will be
 * the result.
 *
 * @param     string    $str1   String A
 * @param     string    $str2   String B
 * @return    string    Merged string
 */ 
function MergeBetween($str1, $str2){

    // Split both strings
    $str1 = str_split($str1, 1);
    $str2 = str_split($str2, 1);

    // Swap variables if string 1 is larger than string 2
    if (count($str1) >= count($str2))
        list($str1, $str2) = array($str2, $str1);

    // Append the shorter string to the longer string
    for($x=0; $x < count($str1); $x++)
        $str2[$x] .= $str1[$x];

    return implode('', $str2);
}

print MergeBetween('abcdef', '__') . "\n";
print MergeBetween('__', 'abcdef') . "\n";
print MergeBetween('bb', 'aa') . "\n";
print MergeBetween('aa', 'bb') . "\n";
print MergeBetween('a', 'b') . "\n";

/*
Output:

a_b_cdef
a_b_cdef
baba
abab
ab
*/