how to change the value of a URL argument?

A modern, simple way to change one query parameter while preserving the others is: read the request parameters into an array, change the single key you need, rebuild the query string with http_build_query(), then output a safe link or redirect. This preserves every other parameter and uses PHP’s built-in encoding rules. (php.net)

Important compatibility and security notes: do not rely on old patterns shown earlier in the thread (bare variables / registerglobals) — that feature was removed from PHP and is no longer available; and avoid POSIX ereg* functions, they were deprecated/removed in modern PHP. Use `preg*for regex work, access inputs through superglobals like$_GET, and always escape HTML output withhtmlspecialchars()and validate/sanitize input (for example withfilter_input()orfilter_var()`). (php.net)

Example (safe, minimal pattern — change only name, keep age and others):

<?php
$params = $_GET;               // explicit, modern access
$params['name'] = 'newName';   // modify single argument
$query = http_build_query($params, '', '&', PHP_QUERY_RFC3986);
$href = htmlspecialchars($_SERVER['PHP_SELF'], ENT_QUOTES, 'UTF-8') . '?' . $query;
echo '<a href="' . $href . '">Send back to main.php</a>';

// or redirect:
// header('Location: ' . $_SERVER['PHP_SELF'] . '?' . $query); exit;
?>

Two quick troubleshooting tips: if the original URL is a full external URL (not the current script) use parse_url() + parse_str() to extract/modify its query before rebuilding; and always choose the appropriate encoding mode (PHP_QUERY_RFC3986 avoids + for spaces). (php.net)

Thanks to @fpepito for the example approach — the workflow is the right idea — and to @chunguy for experimenting; the above updates that pattern to current PHP best practices and adds the safety checks you’ll need.