Session-Related Logic Error

I need another pair of eyes on this problem.  I have two scripts identical except for the instruction on line 34.  One works and one does not.  You can install the code here and run it to see the effect in action.  On my server, the apparent error is that the Session array does not hold the "cv" value if the value was generated by rand().

This one works as expected:

<?php // RAY_temp_session_works.php
error_reporting(E_ALL);

// ALWAYS START SESSION UNCONDITIONALLY
session_start();
if (empty($_SESSION["cv"])) $_SESSION["cv"] = '?';

// SET FORM STRING
$str = <<<'EOCAP'
<pre>
<form method="post">
ENTER 'CCC' IN THIS BOX:
<input name="cv" value="" autocomplete="off" />
<input type="submit" />
</form>
EOCAP;

// WAS THE FORM SUBMITTED?
if (!empty($_POST["cv"]))
{
    // SHOW SESSION AND POST
    echo "<pre>";
    echo "SESSION: ";
    var_dump($_SESSION);
    echo "POST: ";
    var_dump($_POST);

    // TEST FOR MATCH
    if ($_POST["cv"] == $_SESSION["cv"]) echo PHP_EOL . 'MATCH';
    if ($_POST["cv"] != $_SESSION["cv"]) echo PHP_EOL . 'NO MATCH';
}

// GET A NEW FIXED VALUE
$cv = '321';

// STORE THE VALUE IN THE SESSION
$_SESSION["cv"] = $cv;

// AND PRODUCE THE FORM
$new = str_replace('CCC', $cv, $str);
echo $new;
var_dump($_SESSION);

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This one does not work as expected:

<?php // RAY_temp_session_error.php
error_reporting(E_ALL);

// ALWAYS START SESSION UNCONDITIONALLY
session_start();
if (empty($_SESSION["cv"])) $_SESSION["cv"] = '?';

// SET FORM STRING
$str = <<<'EOCAP'
<pre>
<form method="post">
ENTER 'CCC' IN THIS BOX:
<input name="cv" value="" autocomplete="off" />
<input type="submit" />
</form>
EOCAP;

// WAS THE FORM SUBMITTED?
if (!empty($_POST["cv"]))
{
    // SHOW SESSION AND POST
    echo "<pre>";
    echo "SESSION: ";
    var_dump($_SESSION);
    echo "POST: ";
    var_dump($_POST);

    // TEST FOR MATCH
    if ($_POST["cv"] == $_SESSION["cv"]) echo PHP_EOL . 'MATCH';
    if ($_POST["cv"] != $_SESSION["cv"]) echo PHP_EOL . 'NO MATCH';
}

// GET A NEW RANDOM VALUE
$cv = (string)rand(101, 999);

// STORE THE VALUE IN THE SESSION
$_SESSION["cv"] = $cv;

// AND PRODUCE THE FORM
$new = str_replace('CCC', $cv, $str);
echo $new;
var_dump($_SESSION);

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Thanks for your help, ~Ray