Apache does not have write permission on html folder

I installed apache, MySQL, PHP in CentOS 7. All work well, except apache cannot write file in html folder. I found this problem when I used file_put_contents. I tried

  1. Change owner of /var/www to apache:apache, not work
  2. Added Order allow,deny Allow from all to <Directory "/var/www/html"> in httpd.conf, restart httpd service, not work
  3. chmod -R 0777 /var/www/html, not work

What is wrong with my server? Please help me!

UPDATE1: result of ls -al /var/www/html

total 40
drwxrwxrwx. 5 apache apache 4096 Sep 23 10:19 .
drwxr-x---. 4 apache apache 31 Sep 18 01:50 ..
drwxrwxrwx. 15 apache apache 4096 Sep 23 08:43 folder1
drwxrwxrwx+ 10 apache apache 4096 Sep 23 10:19 folder2
-rwxrwxrwx. 1 apache apache 1574 Sep 5 08:55 favicon.ico
-rwxrwxrwx. 1 apache apache 603 Sep 22 05:05 .htaccess
-rwxrwxrwx. 1 apache apache 9841 Sep 8 08:34 index.php
drwxrwxrwx. 8 apache apache 4096 Sep 23 08:44 folder3

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Linear interpolation (lm) in R, weird behavior

Using R 3.2.2, I found a weird behavior running a simple linear interpolation. The first data frame gives the right result :

test<-data.frame(dt=c(36996616, 36996620, 36996623, 36996626), value=c(1,2,3,4))
lm(value~dt, test)$coefficients

  (Intercept)            dt 
-1.114966e+07  3.013699e-01 

By incrementing the dt variable, the coefficient is now NA :

test$dt<-test$dt+1
lm(value~dt, test)$coefficients

(Intercept)          dt 
        2.5          NA 

Any idea why ? Seems there is an overflow somewhere ?

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Where is the PDFsharp documentation?

All, I was trying to learn to use PDFsharp (MigraDoc) to generate PDF. So I tried to find an API document in the official website of PDFsharp to get reference when I have trouble with reading the sample code of PDFsharp. Like Paragraph.AddTab (What does it mean?).
Here it is http://www.pdfsharp.net/ I looking for.

But I am sorry for failing to find this document.

Could someone please show me the link of document? Thanks in advance.

Where is the PDFsharp documentation?