$shortopts  = "";
        $shortopts .= "w:"; // Required value for warning
        $shortopts .= "c:"; // Required value for critical

        $longopts  = array(
                // No long form options
        );

	// Parse our options with getopt
        $options = getopt( $shortopts, $longopts );

        // If slave is x second behind for warning state
        $delayWarn = $options['w'];

        // If slave is x second behind for a critical state
        $delayCritical = $options['c'];

Besides being in a critical or warning state, Nagios also has conditions for normal and unknown. Each state is associated with a status code that will be set upon completion of the script, hence the following associative array:

        // Nagios conditions we can be in
        $statuses = array( 'UNKNOWN' => '-1', 'OK' => '0', 'WARNING' => '1', 'CRITICAL' => '2' );

For the moment, we don't know what condition our replication setup is in. Nor do we have any additional information about the current state, so let's go ahead and define that as such:

        $state = 'UNKNOWN';
        $info = '';

The next step is to go ahead and connect to our slave MySQL instance and query its status using "SHOW SLAVE STATUS;"

		$db = new mysqli( $dbHost, $dbUser, $dbPasswd );

		// Prepare query statement & execute
		$query = $db->prepare( "SHOW SLAVE STATUS" )) {
		$query->execute();

The MySQL query is going to return a number of columns in a single result row. Of immediate concern is if the slave is in error state or not. For that we take a look at the columns labeled Slave_IO_Running, Slave_SQL_Running and Last_Errno.

        // If Slave_IO_Running OR Slave_SQL_Running are not Yes 
        // OR Last_Errno is not 0 we have a problem
        if (( $SlaveIORunning != 'Yes' ) OR ( $SlaveSQLRunning != 'Yes' ) 
        	OR ( $Last_Errno != '0' )) {

            	$state = 'CRITICAL';

If the slave server is not in error, then we'll go ahead and check how far behind it is, and set a warning or critical state given the earlier parameters from the beginning of the script:

        } else if (( $row['Slave_IO_Running'] == 'Yes' ) OR ( $row['Slave_SQL_Running'] == 'Yes' ) OR ( $row['Last_Errno'] == '0' )) {

        	// So far so, good, what about time delay, how behind is the slave database?
			if ( $row['Seconds_Behind_Master'] >= $delayCritical ) {

            	$state = 'CRITICAL';

            } else if ( $row['Seconds_Behind_Master'] >= $delayWarn ) {

            	$state = 'WARN';

            } else {

            	$state = 'OK';

            }

		}

Now that we have determined the state of the secondary database server, we can pass along some information for Nagios to process.

        // What to output?
        switch ( $state ) {

                case "UNKNOWN":
                        $info = 'Replication State: UNKNOWN';
                        break;

                case "OK":
                        $info = 'Replication State: OK Master Log File: ' .$MasterLogFile. ' Read Master Log Position: ' .$ReadMasterLogPos. ' Replication Delay (Seconds Behind Master): ' .$SecondsBehindMaster;
                        break;

                case "WARNING":
                        $info = 'Replication State: WARNING Master Log File: ' .$MasterLogFile. ' Read Master Log Position: ' .$ReadMasterLogPos. ' Replication Delay (Seconds Behind Master): ' .$SecondsBehindMaster;
                        break;

                case "CRITICAL":
                        $info = 'Replication State: CRITICAL Error: ' .$LastErrno. ': ' .$Last_Error. ' Replication Delay (Seconds Behind Master): ' .$SecondsBehindMaster;
                        break;

        }

All that is left is to transfer our information to Nagios via standard out and an exit code:

        // Need to set type to integer for exit() to handle the code properly
        $status = $statuses[$state];
        settype( $status, "integer" );

        fwrite( STDOUT, $info );
        exit( $status );

Putting it all together we get something like this:

#!/usr/bin/php
<?php

	$shortopts  = "";
	$shortopts .= "w:"; // Required value for warning
	$shortopts .= "c:"; // Required value for critical

	$longopts  = array( 
		// No long form options 
	);

	$options = getopt( $shortopts, $longopts );

	// If slave is x second behind, set state as warn
	$delayWarn = $options['w'];

	// If slave is x second behind, set state as critical
	$delayCritical = $options['c'];

	// Nagios conditions we can be in
	$statuses = array( 'UNKNOWN' =----> '-1', 'OK' => '0', 'WARNING' => '1', 'CRITICAL' => '2' );
	$state = 'UNKNOWN';
	$info = '';
	
	$dbUser = 'user';
	$dbPasswd = 'password';
	$dbHost = 'localhost';

	$db = new mysqli( $dbHost, $dbUser, $dbPasswd );

	if ( mysqli_connect_errno() ) {
	
		// Well this isn't good
		$state = 'CRITICAL';
		$info = 'Cannot connect to db server';

	} else {

		// Prepare query statement & execute
		if ( $query = $db->prepare( "SHOW SLAVE STATUS" )) {

			$query->execute();

			// Bind our result columns to variables
			$query->bind_result( $SlaveIOState, $MasterHost, $MasterUser, $MasterPort, $ConnectRetry, $MasterLogFile, $ReadMasterLogPos, $RelayLogFile, $RelayLogPos, $RelayMasterLogFile, $SlaveIORunning, $SlaveSQLRunning, $ReplicateDoDB, $ReplicateIgnoreDB, $ReplicateDoTable, $ReplicateIgnoreTable, $ReplicateWildDoTable, $ReplicateWildIgnoreTable, $LastErrno, $Last_Error, $SkipCounter, $ExecMasterLogPos, $RelayLogSpace, $UntilCondition, $UntilLogFile, $UntilLogPos, $MasterSSLAllowed, $MasterSSLCAFile, $MasterSSLCAPath, $MasterSSLCert, $MasterSSLCipher, $MasterSSLKey, $SecondsBehindMaster, $MasterSSLVerifyServerCert, $LastIOErrno, $LastIOError, $LastSQLErrno, $LastSQLError );

			// Go fetch
			$query->fetch();

			// Done
			$query->close();

			// and done
			$db->close();
	
			// If Slave_IO_Running OR Slave_SQL_Running are not Yes OR Last_Errno is not 0 we have a problem
			if (( $SlaveIORunning != 'Yes' ) OR ( $SlaveSQLRunning != 'Yes' ) OR ( $LastErrno != '0' )) {
		
				$state = 'CRITICAL';	
		
			} else if (( $SlaveIORunning == 'Yes' ) OR ( $SlaveSQLRunning == 'Yes' ) OR ( $LastErrno == '0' )) {
	
				// So far so, good, what about time delay, how behind is the slave database?
	
				if ( $SecondsBehindMaster >= $delayCritical ) {
				
					$state = 'CRITICAL';
				
				} else if ( $SecondsBehindMaster >= $delayWarn ) {
				
					$state = 'WARN';
				
				} else {
	
					$state = 'OK';
		
				}
			
			}
	
	
		} else {
			
			// Well this isn't good
			$state = 'CRITICAL';
			$info = 'Cannot query db server';			
			
		}
	
		// What to output?
		switch ( $state ) {

			case "UNKNOWN":
				$info = 'Replication State: UNKNOWN';
				break;

			case "OK":
				$info = 'Replication State: OK Master Log File: ' .$MasterLogFile. ' Read Master Log Position: ' .$ReadMasterLogPos. ' Replication Delay (Seconds Behind Master): ' .$SecondsBehindMaster;
				break;

			case "WARNING":
				$info = 'Replication State: WARNING Master Log File: ' .$MasterLogFile. ' Read Master Log Position: ' .$ReadMasterLogPos. ' Replication Delay (Seconds Behind Master): ' .$SecondsBehindMaster;
				break;

			case "CRITICAL":
				if ( $info == '' ) {
					
					$info = 'Replication State: CRITICAL Error: ' .$LastErrno. ': ' .$LastError. ' Replication Delay (Seconds Behind Master): ' .$SecondsBehindMaster;
			
				}
			break;
			
		}
	
	}

	// Need to set type to integer for exit to handle the exit code properly
	$status = $statuses[$state];
	settype( $status, "integer" );

	fwrite( STDOUT, $info );
	exit( $status );


?>