System call functions sometimes treat the last line in a multi-line command as a separate command

From manual page: https://php.net/function.exec

    function argon2idKey(string $password, string $salt): string
    {
        $innerCommand =
            'echo ' . escapeshellarg($password) . ' | \\' . "\n"
            . '  argon2 $(echo ' . escapeshellarg(base64_encode($salt)) . ' | base64 -d) -id -l 32 \\' . "\n"
            . '    -t 1 \\' . "\n"
            . '    -m 16 \\' . "\n"
            . '    -p 4 \\ ' . "\n"
            . '    -r';
        $command = 'bash -c ' . escapeshellarg($innerCommand);

        echo $command;

        return exec($command);
    }

The output contains "bash: line 6: -r: command not found". If the output of echo $command; is copied/pasted into the console and ran as-is, it will execute normally. If the -r argument is placed together with -p 4 becoming . ' -p 4 -r', exec() will execute the command with no issues.

Begin with the linked exec() manual page and run the provided PHP/bash reproduction, comparing exec() with the interactive shell behavior. Done means the differing treatment of the final line is explained and the appropriate documentation or implementation follow-up is identified.