Cannot allocate memory: fork: Unable to fork new process in ubuntu

There are three modules related to this settings. 
prefork, worker and event

First find which module is activated using

a2query -M

Now change its conf file in mods-available. 
In following case, prefork is enabled, so make changes in mpm_prefork.conf

MaxConnectionsPerChild
If MaxConnectionsPerChild is 0, then the process will never expire.
Setting MaxConnectionsPerChild to a non-zero value limits the amount of memory that process can consume by (accidental) memory leakage.

MaxRequestWorkers
A simple calculation for MaxRequestWorkers would be: 
(Total Memory – Critical Services Memory) / Size Per Apache process

In this case the server has 3400Mb of memory allocated, 
250Mb used by mySQL and each Apache thread averages about 30Mb. 
MaxRequestWorkers = (3400– 250) / 30
therefore MaxRequestWorkers = 105

/etc/apache2/mods-available/mpm_prefork.conf

 <ifmodule mpm_prefork_module>   
    StartServers          5
    MinSpareServers          5
    MaxSpareServers         10
    MaxRequestWorkers       100
    MaxConnectionsPerChild  1000
</ifmodule>

Restart Apache.

Not able to change permission or execute files in ubuntu

When your file is in NTFS partition, you are not allowed to change permission or execute file.

You need to add permissions for the partition.

Find UUID of all partitions using

sudo blkid

Open file /etc/fstab
and add following line for each partition which should be auto-mounted with executable file permissions.

UUID=12102C02102CEB83 /media/root ntfs-3g auto,users,exec,permissions 0 0

UUID of the partition can be found using above mentioned command and /media/root is the directory where you want the partition should be mounted

Now partition files have changeable and executable permissions.

XAMPP - wrong ELF class: ELFCLASS64

If you have added an extension in your php.ini and you are getting this error while restarting XAMPP or running command  /opt/lampp/bin/php -v as well as as extension is not working, it means you are using 64 bit .so file(extension) in 32 bit XAMPP.

You can get this error while installing ioncube loader .so file, mongo.so file or in similar extension.

Download 32 bit version of .so file and put it in right place in xampp directory.
Mention it in php.ini file and Restart xampp .

Your extension should not be failed this time.