Excellent point ryanpq. Thank you.

Verify web user name

# ps aux | grep apache
apache   17856  0.0  2.6 457444 26508 ?        Sl   11:24   0:04 /usr/sbin/httpd -DFOREGROUND
apache   17893  0.0  2.8 459780 29156 ?        Sl   11:25   0:01 /usr/sbin/httpd -DFOREGROUND
...

# ps aux | grep httpd
apache   17856  0.0  2.6 457444 26508 ?        Sl   11:24   0:04 /usr/sbin/httpd -DFOREGROUND
apache   17893  0.0  2.8 459780 29156 ?        Sl   11:25   0:01 /usr/sbin/httpd -DFOREGROUND
...

Verify owner/permissions for directory

# ls -lhd sigs
drwxr-sr-x. 2 apache apache 4.0K Apr 14 09:29 sigs

To eliminate a path issue, I moved trial.php into the sigs directory and updated the $file variable so it is attempting to write directly within that directory

# cat /var/www/html/sigs/trial.php
<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);
$file = 'file.txt';
echo "file: $file<bR>";
$mf = fopen($file, 'w');
fwrite($mf, 'hi');
fclose($mf);
?>

Output of accessing the file: http://xxxxxx/sigs/trial.php

file: file.txt

Warning: fopen(file.txt): failed to open stream: Permission denied in /var/www/html/sigs/trial.php on line 7

Warning: fwrite() expects parameter 1 to be resource, boolean given in /var/www/html/sigs/trial.php on line 8

Warning: fclose() expects parameter 1 to be resource, boolean given in /var/www/html/sigs/trial.php on line 9

Change permissions for full access

# chmod 0777 /var/www/html/sigs
# ls -lhd /var/www/html/sigs/
drwxrwsrwx. 2 apache apache 4.0K Apr 14 09:29 /var/www/html/sigs/

And try again: http://xxxxx/sigs/trial.php

file: file.txt

Warning: fopen(file.txt): failed to open stream: Permission denied in /var/www/html/sigs/trial.php on line 7

Warning: fwrite() expects parameter 1 to be resource, boolean given in /var/www/html/sigs/trial.php on line 8

Warning: fclose() expects parameter 1 to be resource, boolean given in /var/www/html/sigs/trial.php on line 9

Exact same problem i have. Any solution offer? I think SELinux has controlled this. But not tested. Looking forward for any solution.

I solved a minute ago.
SELinux has authority on writing permits.
Please read accepted answer on this post : https://unix.stackexchange.com/questions/50639/httpd-cant-write-to-folder-file-because-of-selinux

Try this :

chcon -R -t httpd_sys_rw_content_t /path

write your target /path where you want to give write permission.
I don’t know the security hole risk on this commands. Play with your own risk.
Good day.