Test a MySQL replication slave's status

slave_status.php

#!/usr/local/bin/php
<?php

$error = false;
$errno = 0;

try {
    $db = new PDO('mysql:dbname=stations;host=localhost', 'root', 'password');
    $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    /** @var $slave_status StdClass */
    $slave_status = $db->query('show slave status')->fetchObject();
} catch (PDOException $e) {
    $error = "Error getting slave status";
    $errno = 20;
}

if (!$errno) {
    if (!$slave_status) {
        $error = "Empty slave status response";
        $errno = 1; // Probably not a slave
    } elseif (!property_exists($slave_status, 'Slave_IO_Running')
        || !property_exists($slave_status, 'Slave_SQL_Running')
        || !property_exists($slave_status, 'Seconds_Behind_Master')
    ) {
        $error = "Expected vars in slave status response missing";
        $errno = 21; // Shouldn't happen
    } elseif ($slave_status->Slave_IO_Running !== 'Yes') {
        $error = "Slave IO thread stopped";
        $errno = 2; // Probably not a slave
    } elseif ($slave_status->Slave_SQL_Running !== 'Yes') {
        $error = "Slave IO running but slave SQL thread stopped";
        $errno = 10; // Probably a slave that's in trouble
    } elseif (!preg_match('/^\d+$/', $slave_status->Seconds_Behind_Master)) {
        $error = "Seconds behind is NaN";
        $errno = 11; // Probably a slave that's in trouble
    } elseif ($slave_status->Seconds_Behind_Master > 99) {
        $error = "Excessive slave lag: $slave_status->Seconds_Behind_Master";
        $errno = 12; // Probably a slave that's in trouble
    }
}

if ($errno) {
    if ($errno >= 20) {
        echo "MySQL server is in trouble. ";
    } elseif ($errno >= 10) {
        echo "Replication slave server is in trouble. ";
    } else {
        echo "Server is probably not supposed to be a slave. ";
    }
    fwrite(STDERR, "$error ($errno)\n");
    exit($errno);
}