fs.php: add concat_path

inc/functions/fs.php

@@ -15,3 +15,17 @@ function move_or_copy_uploaded(string $from, string $to) {
 	}
 	return true;
 }
+
+/**
+ * Concats two paths accounting for directory separators.
+ */
+function concat_path(string $base, string $other): string {
+	if ($base[\strlen($base) - 1] === DIRECTORY_SEPARATOR) {
+		$base = \substr($base, 0, \strlen($base) - 1);
+	}
+	if ($other[0] === DIRECTORY_SEPARATOR) {
+		return $base . $other;
+	} else {
+		return $base . DIRECTORY_SEPARATOR . $other;
+	}
+}