The lock-file mechanism in Kevin Trass's note is incorrect because it is subject to race conditions.

For locks you need an atomic way of verifying if a lock file exists and creating it if it doesn't exist. Between file_exists and file_put_contents, another process could be faster than us to write the lock.

The only filesystem operation that matches the above requirements that I know of is symlink().

Thus, if you need a lock-file mechanism, here's the code. This won't work on a system without /proc (so there go Windows, BSD, OS X, and possibly others), but it can be adapted to work around that deficiency (say, by linking to your pid file like in my script, then operating through the symlink like in Kevin's solution).

#!/usr/bin/php
<?php

define('LOCK_FILE', "/var/run/" . basename($argv[0], ".php") . ".lock");

if (!tryLock())
    die("Already running.\n");

# remove the lock on exit (Control+C doesn't count as 'exit'?)
register_shutdown_function('unlink', LOCK_FILE);

# The rest of your script goes here....
echo "Hello world!\n";
sleep(30);

exit(0);

function tryLock()
{
    # If lock file exists, check if stale.  If exists and is not stale, return TRUE
    # Else, create lock file and return FALSE.

    if (@symlink("/proc/" . getmypid(), LOCK_FILE) !== FALSE) # the @ in front of 'symlink' is to suppress the NOTICE you get if the LOCK_FILE exists
        return true;

    # link already exists
    # check if it's stale
    if (is_link(LOCK_FILE) && !is_dir(LOCK_FILE))
    {
        unlink(LOCK_FILE);
        # try to lock again
        return tryLock();
    }

    return false;
}
?>

All of the examples above require you to have shell command execution turned on- this example uses only PHP functions and should work on any system (posix is included by default)-

the key is posix_getsid which will return FALSE if the processes does not exist.

<?php 
$lockfile = sys_get_temp_dir() . '/myScript.lock'; 
$pid = file_get_contents($lockfile); 
if (posix_getsid($pid) === false) { 
   print "process has died! restarting...\n"; 
   file_put_contents($lockfile, getmypid()); // create lockfile 
} else { 
   print "PID is still alive! can not run twice!\n"; 
   exit; 
} 
?> 

:-) perfect if you need to make sure a cron job or shell script has ended before it can be started again.

This works across users- if the cron job is started as 'root' your 'web user' can see if the process is still alive (useful for system status pages)

Looking to create a lock-file mechanism for a cmd-line script?

Enjoy!

#!/usr/bin/php
<?php

define( 'LOCK_FILE', "/var/run/".basename( $argv[0], ".php" ).".lock" );
if( isLocked() ) die( "Already running.\n" );

# The rest of your script goes here....
echo "Hello world!\n";
sleep(30);

unlink( LOCK_FILE );
exit(0);

function isLocked()
{
    # If lock file exists, check if stale.  If exists and is not stale, return TRUE
    # Else, create lock file and return FALSE.

    if( file_exists( LOCK_FILE ) )
    {
        # check if it's stale
        $lockingPID = trim( file_get_contents( LOCK_FILE ) );

       # Get all active PIDs.
        $pids = explode( "\n", trim( `ps -e | awk '{print $1}'` ) );

        # If PID is still active, return true
        if( in_array( $lockingPID, $pids ) )  return true;

        # Lock-file is stale, so kill it.  Then move on to re-creating it.
        echo "Removing stale lock file.\n";
        unlink( LOCK_FILE );
    }
    
    file_put_contents( LOCK_FILE, getmypid() . "\n" );
    return false;

}
?>

My lockfile system

<?php
function isLocked(){
    if(file_exists(LOCK_FILE)) {
        $lockingPID = trim(file_get_contents(LOCK_FILE)); 
        $test=trim(`ps -p $lockingPID -o pid=`);
        if(!empty($test)) return true; 
        echo "Removing stale lock file.\n"; 
        unlink(LOCK_FILE);
    }
    file_put_contents(LOCK_FILE, getmypid()."\n"); 
    return false;
}
?>

On windows, ps is not available. Instead, to view a list of running processes, you can use exec('tasklist'); To kill processes you can use exec('taskkill); Enter taskkill /? for more information.

One good use for this is deciding on a concurrency-safe temporary file or directory name. You can be assured that no two processes on the same server have the same PID, so this is enough to avoid collisions. For example:

<?php
$tmpfile = "/tmp/foo_".getmypid();
// Use $tmpfile...
// Use $tmpfile...
// Use $tmpfile...
unlink ($tmpfile);
?>

If you are sharing /tmp over the network (which is odd....) then you can, of course, mix in the PHP server's IP address.

On Linux you can check if a process is still running by verifying if the PID exists in the /proc directory :

<?php
if(file_exists('/proc/'.$pid))
{
    echo 'The process is still running.';
}
?>

<?php
/*

mixed getpidinfo(mixed pid [, string system_ps_command_options])

this function gets PID-info from system ps command and return it in useful assoc-array,
or return false and trigger warning if PID doesn't exists

$pidifo=getpidinfo(12345);

print_r($pidifo);

Array
(
    [USER] => user
    [PID] => 12345
    [%CPU] => 0.0
    [%MEM] => 0.0
    [VSZ] => 1720
    [RSS] => 8
    [TT] => ??
    [STAT] => Is
    [STARTED] => 6:00PM
    [TIME] => 0:00.01
    [COMMAND] => php someproces.php > logfile
)

*/

//////////////////////////////////////////////

function getpidinfo($pid, $ps_opt="aux"){

   $ps=shell_exec("ps ".$ps_opt."p ".$pid);
   $ps=explode("\n", $ps);
   
   if(count($ps)<2){
      trigger_error("PID ".$pid." doesn't exists", E_USER_WARNING);
      return false;
   }

   foreach($ps as $key=>$val){
      $ps[$key]=explode(" ", ereg_replace(" +", " ", trim($ps[$key])));
   }

   foreach($ps[0] as $key=>$val){
      $pidinfo[$val] = $ps[1][$key];
      unset($ps[1][$key]);
   }
   
   if(is_array($ps[1])){
      $pidinfo[$val].=" ".implode(" ", $ps[1]);
   }
   return $pidinfo;
}

?>

We also had this challenge in our company to prevent a php script in a cron job from overlapping each other. 

We made this solution

<?php
        // Initialize variables
    $found            = 0;
    $file                 = basename(__FILE__);
    $commands    = array();

        // Get running processes.
    exec("ps w", $commands);

        // If processes are found
    if (count($commands) > 0) {

        foreach ($commands as $command) {
            if (strpos($command, $file) === false) {
                               // Do nothin'
            }
            else {
                               // Let's count how many times the file is found.
                $found++;
            }
        }
    }

        // If the instance of the file is found more than once.
    if ($found > 1) {
        echo "Another process is running.\n";
        die();
    }

        /**
        * 
        * Regular process here...
        * 
        */
?>

Here is how you'd use exec on a posix system to accomplish counting processes quickly.

I want to know how many processes are running with 'update.php' in the command:

ps aux|grep "[u]pdate.php"|wc -l

(the trick of using [u]pdate.php instead of update.php makes sure that the grep command itself is not matched).  Be sure to use quotes in the command, or it won't work either.

So, the code:

<?php
function countProcesses($scriptName)
{
    // ps aux|grep "[u]pdate.php"|wc -l
    $first    =    substr($scriptName, 0, 1);
    $rest    =    substr($scriptName, 1);
    
    $name    =    '"['.$first.']'.$rest.'"';
    $cmd    =    "ps aux | grep $name | wc -l";
            
    $result    =    exec($cmd);
            
    return $result;
}
?>

The 'ps' command has an option that can make filtering for a specific process more efficient.  Using this the work of looking for matching processes can be made neater:

<?php
    /*
    Return an array of the pids of the processes that are running for the specific command
    e.g.
        returnPids('myprocess.php');
    */
    function returnPids($command) {
        exec("ps -C $command -o pid=",$pids);
        foreach ($pids as $key=>$value) $pids[$key]=trim($value);
        return $pids;
    }    

    /*
    Returns an array of the pids for processes that are like me, i.e. my program running
    */
    function returnMyPids() {
        return returnPids(basename($_SERVER["SCRIPT_NAME"]));
    }
?>

e.g. to bomb out if I'm running already

if (count(returnMyPids())>1) exit;

On windows, you can get a list of PID's using this single line statement: <?php $pids = array_column(array_map('str_getcsv', explode("\n",trim(`tasklist /FO csv /NH`))), 1); ?>.

You can use this function also to avoid more than one instance of your app.

You can also use this class.
http://www.pure-php.de/node/20

Usage:

<?php

inlude("ProcessHandler.class.php");

if(ProcessHandler::isActive()){
   die("Already running!\n";);
}else{
   ProcessHandler::activate();
   //run my app
}

?>

Based on what james at voodoo dot co dot uk said, but modified for CLI scripts (ie: there is no $_SERVER). 

<?php

/**
 * Check for a current process by filename
 * @param $file[optional] Filename
 * @return Boolean
 */
function processExists($file = false) {

    $exists     = false;
    $file       = $file ? $file : __FILE__;

    // Check if file is in process list
    exec("ps -C $file -o pid=", $pids);
    if (count($pids) > 1) {
        $exists = true;
    }
    return $exists;
}

?>