PHP / MySQL / PDO - the result is false, but there is no DB error message?

I have a piece of code that works weird. Sometimes it works, sometimes it doesn’t. The server has an absolutely ancient copy of PHP (5.1.6, five years, but with security fixes that were manually included by Red Hat).

Here's the code, including the debug lines I currently have:

<?php
// Includes json_print, which does a json_encode, an 
// appropriate content-type header, prints it, and exits the script.
include_once('json-functions.php');

$uid = $_POST['uid'];

$salted = false;

if(isset($_POST['salted'])){ $salted = true; }

// No uid given.
if(is_null($uid) || $uid === ''){
        $details = array(
            'error' => 1,
            'errorMessage' => 'No unique ID entered.  Please try again.',
        );
        json_print($details);
}

// Validate uid.  Must be 64 digit hexadecimal value.
$pattern = '/^[a-f0-9]{64}$/i';
if(preg_match($pattern, $uid) === 0){
        $details = array(
            'error' => 2,
            'errorMessage' => 'Invalid unique ID.',
        );
        json_print($details);
}


include_once('../db.php');

header('Content-Type: text/plain');

// Is this salted already?
var_dump($salted);

// What the UID?
var_dump($uid);

if(!$salted){ $uid = hash('sha256', $salt.$uid); }

// Was the UID salted?  (shouldn't be double-salted)
var_dump($uid);

// The query
$SQL = 'SELECT ';

    $SQL .= 'p.patronID AS patronID, ';
    $SQL .= 'uniqueID, ';
    $SQL .= 'status, ';
    $SQL .= 'active, ';
    $SQL .= 'd.name AS department, ';
    $SQL .= 'docdelivery, ';
    $SQL .= 'terms, ';
    $SQL .= 'copyright, ';
    $SQL .= 'lastLogin, ';
    $SQL .= 'updated, ';
    $SQL .= 'TIMESTAMPDIFF(MINUTE, lastLogin, NOW()) AS recency, ';
    $SQL .= 'DATEDIFF(NOW(), updated) AS stale, ';

    $SQL .= 'AES_DECRYPT(first, ?) AS first, ';
    $SQL .= 'AES_DECRYPT(last, ?) AS last, ';
    $SQL .= 'AES_DECRYPT(barcode, ?) AS barcode, ';
    $SQL .= 'INET_NTOA(AES_DECRYPT(ip, ?)) AS ip, ';
    $SQL .= 'AES_DECRYPT(email, ?) AS email, ';
    $SQL .= 'AES_DECRYPT(phone, ?) AS phone, ';
    $SQL .= 'AES_DECRYPT(address1, ?) AS address1, ';
    $SQL .= 'AES_DECRYPT(address2, ?) AS address2, ';
    $SQL .= 'AES_DECRYPT(city, ?) AS city, ';
    $SQL .= 'AES_DECRYPT(state, ?) AS state, ';
    $SQL .= 'AES_DECRYPT(zip, ?) AS zip ';

$SQL .= 'FROM patrons p, departments d ';
$SQL .= 'WHERE department = d.deptID ';
$SQL .= 'AND uniqueID = ?';

$query = $DB->prepare($SQL);

$p = array(
    $key,
    $key,
    $key,
    $key,
    $key,
    $key,
    $key,
    $key,
    $key,
    $key,
    $key,
    $uid,
);

$query->execute($p);

$result = $query->fetch();

// dump the results
var_dump($result);
print "\n\n";

// And any error info
var_dump($DB->errorInfo());
exit;

Here is an example output when it works correctly:

// Salted is true
bool(true)

// UID is:
string(64) "52223d99e1db275716028cf6fd4f58895b1df7eb8e061cefab346b8ce3cf4ff4"

// It was not double-salted:
string(64) "52223d99e1db275716028cf6fd4f58895b1df7eb8e061cefab346b8ce3cf4ff4"

// Results were:
array(46) {
  ["patronID"]=>
  string(1) "126"
  [0]=>
  string(1) "126"
  ["uniqueID"]=>
  string(64) "52223d99e1db275716028cf6fd4f58895b1df7eb8e061cefab346b8ce3cf4ff4"
  [1]=>
  string(64) "52223d99e1db275716028cf6fd4f58895b1df7eb8e061cefab346b8ce3cf4ff4"
  ["status"]=>
  string(1) "4"
  [2]=>
  string(1) "4"
  ["active"]=>
  string(1) "1"
  [3]=>
  string(1) "1"

  *** snip! ***

  [21]=>
  string(2) "TX"
  ["zip"]=>
  string(5) "78623"
  [22]=>
  string(5) "78623"
}

// Errors reported?
array(1) {
  [0]=>
  string(5) "00000"
}

And here is an example output when it fails:

 // Salted is true
bool(true)

// UID is:
string(64) "1d6fa3b897b07301a836f5441d23f60e7cb4b52a00ee6d20648fe51b01c769bf"

// It was not double salted
string(64) "1d6fa3b897b07301a836f5441d23f60e7cb4b52a00ee6d20648fe51b01c769bf"

// Results were:
bool(false)

// Error code was:
array(1) {
  [0]=>
  string(5) "00000"
}

I can’t understand why it works for SOME uids, but not for others. In addition, in the second example, the result set appears FALSE, but the database reports error 00000, which means "no errors." I already checked whether the request was prepared correctly, and this is in both cases.

What am I missing here?