How do I log errors and warnings into a file?

Use the following code:

ini_set("log_errors", 1);
ini_set("error_log", "/tmp/php-error.log");
error_log( "Hello, errors!" );

Then watch the file:

tail -f /tmp/php-error.log

Or update php.ini as described in this blog entry from 2008.

See

• error_log — Send an error message somewhere

Example

error_log("You messed up!", 3, "/var/tmp/my-errors.log");

You can customize error handling with your own error handlers to call this function for you whenever an error or warning or whatever you need to log occurs. For additional information, please refer to the Chapter Error Handling in the PHP Manual

Simply put these codes at top of your PHP/index file:

error_reporting(E_ALL); // Error/Exception engine, always use E_ALL

ini_set('ignore_repeated_errors', TRUE); // always use TRUE

ini_set('display_errors', FALSE); // Error/Exception display, use FALSE only in production environment or real server. Use TRUE in development environment

ini_set('log_errors', TRUE); // Error/Exception file logging engine.
ini_set('error_log', 'your/path/to/errors.log'); // Logging file path

Add this code in file .htaccess (as an alternative to file php.ini or the ini_set function):

<IfModule mod_php5.c>
php_flag log_errors on 
php_value error_log ./path_to_MY_PHP_ERRORS.log
# php_flag display_errors on 
</IfModule>

• as commented: this is for Apache-type servers, and not for Nginx or others.

That's my personal short function

# logging
/*
[2017-03-20 3:35:43] [INFO] [file.php] Here we are
[2017-03-20 3:35:43] [ERROR] [file.php] Not good
[2017-03-20 3:35:43] [DEBUG] [file.php] Regex empty

mylog ('hallo') -> INFO
mylog ('fail', 'e') -> ERROR
mylog ('next', 'd') -> DEBUG
mylog ('next', 'd', 'debug.log') -> DEBUG file debug.log
*/
function mylog($text, $level='i', $file='logs') {
    switch (strtolower($level)) {
        case 'e':
        case 'error':
            $level='ERROR';
            break;
        case 'i':
        case 'info':
            $level='INFO';
            break;
        case 'd':
        case 'debug':
            $level='DEBUG';
            break;
        default:
            $level='INFO';
    }
    error_log(date("[Y-m-d H:i:s]")."\t[".$level."]\t[".basename(__FILE__)."]\t".$text."\n", 3, $file);
}

Take a look at the log_errors configuration option in php.ini. It seems to do just what you want to. I think you can use the error_log option to set your own logging file too.

When the log_errors directive is set to On, any errors reported by PHP would be logged to the server log or the file specified with error_log. You can set these options with ini_set too, if you need to.

(Please note that display_errors should be disabled in php.ini if this option is enabled)

None of these answers mention the importance or relevance to Agile teams; professional-level development work is more often than not done within the context of a squad.

Keeping in mind that many development teams use a task-tracking tool such as JIRA, I prefer to use a timestamp to log each error as a separate file:

error_reporting(E_ALL);
ini_set('log_errors', 1);
ini_set('error_log', $_SERVER['DOCUMENT_ROOT'] . '/logs/php/php-errors-' . time() . '.txt');

Using this approach, it is easy for a Project Manager or Team Lead to then isolate each error as a specific file that can then be attached to a unique JIRA defect, assigned to a developer, and tracked (the timestamp can be linked within the defect to easily track while the defect fix is In-Progress). When the issue is closed, then the file can be deleted from the log directory, while a copy of it (if needed for future reference) can be accessed in the JIRA.

To log all errors in a file, including Fatal error 500, we are forced to use the php.ini mechanism, as PHP itself could not handle such fatal errors. Generic answer for 2024:

Log PHP errors:

Example is production safe (won't display errors on the webpage), given for PHP.8.3, on Debian 12, with the PHP-FPM user being www-data (This the default):

nano /etc/php/8.3/fpm/php.ini

Add to the bottom:

log_errors = on
error_log = /var/log/php8.3-fpm.log

Give ownership to that log file to the www-data user:

chown www-data:www-data /var/log/php8.3-fpm.log

Allow writing to that log file by the owner:

chmod 660 /var/log/php8.3-fpm.log

Restart PHP-FPM service:

sudo systemctl restart php8.3-fpm

Read PHP potential errors logs in realtime:

tail -f /var/log/php8.3-fpm.log

Optionnal: Log connections times

Edit the PHP-FPM configuration file:

nano /etc/php/8.3/fpm/pool.d/www.conf

Add this to the bottom:

php_admin_value[error_log] = /var/log/php8.3-fpm.log
php_admin_flag[log_errors] = on

listen = /run/php/php8.3-fpm.sock
listen.owner = www-data
listen.group = www-data
listen.mode = 0660
user = www-data
group = www-data

Restart PHP-FPM service:

sudo systemctl restart php8.3-fpm

Optionnal: Write in that log from PHP.

fwrite(fopen("/var/log/php8.3-fpm.log", "w"), json_encode($MYVAR));

Optionnal: Write the remote IP adress, from PHP.

fwrite(fopen("/var/log/php8.3-fpm.log", "w"), "New connection from: " .$_SERVER["REMOTE_ADDR"] . " \n");

In addition, you need the "AllowOverride Options" directive for this to work. (Apache 2.2.15)

here's my log function:

You can edit the log rows by editing $maxLogs=5, also the order to write your logs $logOrder='top'

<?php
lg('script start','start');

#Code......
lg('script end','End of code');

function lg($str,$mod='Your Log Category'){
    $ts = microtime(true);
    if(!defined('logTimer')){
        define('logTimer',microtime(true));
    }
    $diff=abs(round(($ts-logTimer)*1000,2));
    $maxLogs=5;
    $logOrder='top';#new Logs at top
    
    
    $filename = './log.txt';
    $log=[];
    if(!file_exists($filename)){
        if(!file_put_contents($filename,json_encode($log,128))){
            echo "Can’t open to write '$filename' Check Permissions";
            return;
        }
    }else{
        $c=file_get_contents($filename);
        if(trim($c)==''){$c='[]';}
        
        $log =@json_decode($c,true);
        if(!is_Array($log)){$log=[];}
    }
    $new=['mod'=>$mod,'date'=> date('Y-m-d H:i:s')." Scripttime: ".$diff."ms",'log'=>$str];
    if($logOrder=='top'){
        array_unshift($log , $new);
        $log=array_slice($log,0,$maxLogs);
        }else{
        $log[]=$new;
        $log=array_slice($log,0-$maxLogs,$maxLogs);
    }
    
    
    $logs=json_encode($log,128);
    if(!file_put_contents($filename,$logs) ){echo ("Can’t open to write '$filename' Check Permissions") ;return;}
    return $str;
}
?>

The Output looks like:

[
    {
        "mod": "delete",
        "date": "2022-08-04 13:48:02 0.33ms",
        "log": "test 2"
    },
    {
        "mod": "start",
        "date": "2022-08-04 13:48:29 0ms",
        "log": "test"
    },
    {
        "mod": "delete",
        "date": "2022-08-04 13:48:29 0.27ms",
        "log": "test 2"
    },
    {
        "mod": "start",
        "date": "2022-08-04 13:48:34 0ms",
        "log": "test"
    },
    {
        "mod": "delete",
        "date": "2022-08-04 13:48:34 0.92ms",
        "log": "test 2"
    }
]