Bash: function in a script with flags

Using https://stackoverflow.com/a/7948533/31298396, we can implement flags for a script testing as follows:

#!/bin/bash
# Flags
    # Source: https://stackoverflow.com/a/7948533/31298396
TEMP=$(getopt -o ''\
              --long foo: \
              -n 'testing' -- "$@")

if [ $? != 0 ] ; then echo "Terminating..." >&2 ; exit 1 ; fi

# Note the quotes around '$TEMP': they are essential!
eval set -- "$TEMP"

while true; do
  case "$1" in
    --foo ) :; shift 2 ;;
    -- ) shift; break ;;
    * ) break ;;
  esac
done

test_function () {
    echo "$1"
}

test_function "$1"

Indeed, running ./testing "Hello" prints Hello as expected --- I tested this in both WSL and a GitHub Codespace running Debian. But running ./testing --foo "Hello" gives empty output. Any idea how to fix this?