#!/bin/bash
sed "s/\$_REQUEST\['old'\]/\$_REQUEST\['new'\]/g" <<'EOF'
$_REQUEST['old']
$_REQUEST['old'] == ?#@?
lala
EOF
IyEvYmluL2Jhc2gKCgpzZWQgInMvXCRfUkVRVUVTVFxbJ29sZCdcXS9cJF9SRVFVRVNUXFsnbmV3J1xdL2ciIDw8J0VPRicKCiRfUkVRVUVTVFsnb2xkJ10KICAgJF9SRVFVRVNUWydvbGQnXSA9PSA/I0A/CiAgIAogbGFsYQogCkVPRgog