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<?php 

$adding  = 120;

$path    = '.1.2.3.4.';
$trimmed = trim($path, '.');
$paths   = explode('.', $trimmed);

// Important: Should be passing reference!
array_walk($paths, function(&$item) use ($adding) {
  $item += $adding;
});

$newPath = '.'.implode('.', $paths).'.';

var_export($newPath);  // .121.122.123.124.