PHP 相对路径转换为绝对路径 realpath

* 相对路径 -> 绝对路径 realpath

<?php
/**
 * @param string $in_rel: relative directory
 * @param string $out_abs: absolute directory
 */
define('PATH_MAX', 255);
function sub_rel2abs(string $in_rel, string &$out_abs) {
    $i_rtn = 0;  // return value
    $ss_rel = "";  // for relative path build
    $st_fpos = 0;    // front separator index
    $sv_path = [];   // divide path to array

    $st_pos = strpos($in_rel, DIRECTORY_SEPARATOR);
    $npos = 0;
    while ($npos != $st_pos) {
        if ($st_pos != 0) {
            array_push($sv_path, substr($in_rel, $st_fpos, $st_pos - $st_fpos));
        }
// next...
        $st_fpos = $st_pos;   // set current pos to last pos
        $st_pos++;            // from next index
        $st_pos = strpos($in_rel, DIRECTORY_SEPARATOR, $st_pos);  // next separator index
    } // while ( $npos != $st_pos )
// final separator
    array_push($sv_path, substr($in_rel, $st_fpos));

    $lpc = 0;    // loop count
    $i_max = count($sv_path);
    while ($lpc < $i_max && 0 === $i_rtn) {
        $ss_rel .= $sv_path[$lpc];
// relative path => relative path
        $c_abs = realpath($ss_rel);
        if ($c_abs === false) {
            $i_rtn = -1;
        } else {
            $ss_rel = $c_abs;
            $i_rtn = 0;
        }
        $lpc++;
    } // while (count($sv_path)>0)

// normal ending
    if (0===$i_rtn) {
        $out_abs = $ss_rel;  // set converted path
    }
    return $i_rtn;
}

// test
$inDir = "/Users/Mch/Code/php/Directory";
is_dir($inDir) || mkdir($inDir, 0777, true);

$wd = __DIR__;
chdir($inDir);

$out = "";
echo sub_rel2abs("../../../eclipse-workspace/blog.zip", $out).PHP_EOL;
echo $out.PHP_EOL;

chdir($wd);
@rmdir($inDir);

output:

0
/Users/Mch/eclipse-workspace/blog.zip

　　这里直接realpath就可以了，为什么多此一举?

*  绝对路径 -> 相对路径

   ... [TO BE DONE]