Create html file with php script

if(! file_exists ($filename))
  {
  $fp = fopen($filename, 'w');
  fwrite($fp,  $file);
  fclose($fp);
  }

I'm not familiar with the method you're using called 'site_url'. From a quick google search it looks like it's a method in Word Press. Ignoring the site_url method, you might want to test using a specific url, like http://ted.com for example. I've specified is_file rather than file_exists because file_exists will return true even if the path you've specified is a directory, whereas is_file will only return true if the path is an actual file. Try this code, setting the $site variable to a site url or path to a file.

I've also switched some code around, doing a check first to see if the file exists before attempting to read in the contents of $site. This way, if the file already exists, you're not needlessly reading in the contents of $site.

$filename = "Service_delivery_report_" . date("Y-m-d",
                                              time()). ".html";

$filepath = realpath("./") . "/views/sd_reports/" . $filename;

if (!is_file($filepath))
{
    $site = "http://somesite.com/somepage";

    if ($content = file_get_contents($site))
    {   
        file_put_contents($filepath,
                          $content);    
    }
    else
    {
        echo "Could not grab the contents of some site";
    }
}

Use file_exists();

<?php
$filename = '/path/to/foo.txt';

if (file_exists($filename)) {
    echo "The file $filename exists";
} else {
    echo "The file $filename does not exist";
}
?>

Then, if it doesn't exist, create a file with fopen();, like so:

$handle = fopen($filename, "w");

Try this

$file = file_get_contents(site_url('appraisal/createReport'));      
$filename = 'Service_delivery_report_'.date('Y-m-d', time()).'.html';
if(! file_exists ($filename))
{
$f = fopen($filename, "w");
fwrite($f,  $file);
fclose($f);
}
else
echo "file already exist";