Try this slightly modified code in your DocumentRoot/.htaccess
:
Options +FollowSymLinks -Multiviews
RewriteEngine On
RewriteRule ^channels/page/([0-9]+)/?$ channels/index.php?p=$1 [L]
RewriteRule ^channels/([_0-9a-z-]+)/?$ channels/index.php?o=$1 [L]
RewriteRule ^channels/([_0-9a-z-]+)/([0-9]+)/?$ channels/index.php?o=$1&p=$2 [L]
# User profiles
RewriteRule ^user/([^/]*)/$ /user/index.php?usr=$1 [L]
# Check to see if the URL points to a valid file
RewriteCond %{REQUEST_FILENAME} !-f
# Trailing slash check
RewriteCond %{REQUEST_URI} !(.*)/$
# Add slash if missing & redirect
RewriteRule ^(.*)$ $1/ [L,R=301]
# Delete ".php" extension and adds "/"
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{THE_REQUEST} \s/[^?\s]+\.php
RewriteRule (.*)\.php$ /$1/ [L,R=301]
RewriteCond %{REQUEST_URI} ^(.+)/$
RewriteCond %{DOCUMENT_ROOT}%1.php -f
RewriteRule (.*)/$ $1.php [L]
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI}.php -f
RewriteRule [^/]$ %{REQUEST_URI}/ [L,R=301]