Cron job: Code you cannot have PHP - php

Question

My cron job does not work and I was wandering if there is any PHP code which will stop it from forming.
I know how it works, but I want to know what kind go code will not read in PHP code.
One code I found out does not work is sessionstart() and $_SESSION because it saves it on the server like a cookie and cron job does not go through the server or something.
Therefore, is there any other code which will not work from your own knowledge like maybe file_get_contents or fopen() (I don't know if these don't work. Just giving ideas)
EDIT
<?php
$dbhost = "localhost";
$dbuser = "it32_2015";
$dbpass = "it32_2015";
$conn = mysql_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
die('Could not connect: ' . mysql_error());
}
$sql = 'SELECT Title, Pubdate, Link FROM ytable';
mysql_select_db('it32_2015');
$retval = mysql_query( $sql, $conn );
if(! $retval )
{
die('Could not get data: ' . mysql_error());
}
while($row = mysql_fetch_array($retval, MYSQL_ASSOC))
{
$status .= "速報ニュース：".
"{$row['Title']} : ".
"ここでチェック！".
"{$row['Link']}".
"時間：".
"{$row['Pubdate']}".
" #yahoonews END";
}
$statusarray = explode(" END",$status);
$result = mysql_query('SELECT MIN(ID) AS min, MAX(ID) AS max FROM ytable') or exit(mysql_error());
$row = mysql_fetch_assoc($result);
$check = file_get_contents('/home/www2/it32.lady2.itall.co.jp/www/counter.php');
if($check < $row['max']){
if (is_numeric($check)){
$counter = ++$check;
}
}
$fp = fopen('/home/www2/it32.lady2.itall.co.jp/www/counter.php',"w+");
fwrite($fp, $counter);
fclose($fp);
require("/home/www2/it32.lady2.itall.co.jp/www/tweet.php");
?>
Is there any code I cannot use for cron in my code.

Answer 1

Cron jobs i.e. scheduled tasks are scripts that run periodically. As such, the script isn't meant to be interfaced with by a user. It is just meant to carry out an action. So the reason sessions don't work with cron jobs is because there is no user to start a session for. Any PHP function that requires a user to work (e.g. setcookie(), session_start()) will not work. All other functions will.
Edit:
All the functions in your script will work as intended.