My plugin code appears not to execute

[Simetrical]

In my forum, every time you give reputation it's only one point, so the default hardcoded values of 100-200 rep points per icon are unacceptable. On 3.0.x I just hacked the code, but now I've upgraded to 3.6 and am trying to use a plugin. I used the reputation_image hook, but nothing whatsoever that I put there seems to be executed. If, e.g., I add an echo statement at the very top, it doesn't do anything. I've confirmed that the plugin is active and plugins in general are enabled. Here's the exact code I've entered into "Plugin PHP code" at the moment:

PHP Code:

echo "<!-- This is working -->";
define('REP_INCREMENT', 10);
if ($post['showreputation'] OR !($perms['genericpermissions'] & $vbulletin->bf_ugp_genericpermissions['canhiderep'])) {
    $reputation_value = $post['reputation'];
    $post['reputationdisplay'] = '';
    if (-REP_INCREMENT < $reputation_value && $reputation_value < REP_INCREMENT) {
        $reputationgif = 'balance';
    } elseif ($reputation_value >= REP_INCREMENT) {
        $reputationgif = 'pos';
        $reputationhighgif = 'highpos';
    } else {
        $reputationgif = 'neg';
        $reputationhighgif = 'highneg';
        $reputation_value *= -1;
    }
    if ($reputation_value > 5*REP_INCREMENT) {
        // highpos/highneg count double
        $reputation_value = 2.5*REP_INCREMENT) + $reputation_value/2;
    }
    // award 1 reputation bar for every REP_INCREMENT points
    $reputationbars = intval($reputation_value / REP_INCREMENT);
    for ($i = 0; $i <= $reputationbars; $i++) {
        $posneg = $i >= 5 ? $reputationhighgif : $reputationgif;
        $post['level'] = $vbphrase['reputation' . $post['reputationlevelid']];
        eval('$post[\'reputationdisplay\'] .= "' . fetch_template('postbit_reputation') . '";');
    }
} 

The echo text doesn't appear, and the rest also seems to do nothing. Is there something I'm missing? Thanks in advance for any help.