php fails to write large text files (drop junk chars) - php

Question

I have a php script that create a large text file.
The text file contains php code, with variables and data
e.g.
$myvar[1]='some value 1';
$myvar[2]='some value 2';
$mynum[1]=10;
$mynum[2]=11;
I use that file in the next run, as include file. Because of data complexity, amound of variables, etc, I have better performance when I get these data directly as included in my code versus if I get these data from a database.
So, the next run of script, get the previu's run data, do some calcs and create the data file (php code) for the next run.
I use fopen, fwrite, fclose , to write one big string that contains all the variables.
My files are about 800KB large. The problem is (not in every run) the file contains junk chars that I never write, file stop before the normal end, or I have a missing part.
eg (junk chars)
$myvar[1]='some value 1';
$myvar[2]='some value 2';
$mynum[1]=10;
$mynum[2**]]'**]=11;
eg (wrong end)
$myvar[1]='some value 1';
$myvar[2]='some value 2';
$mynum[1]=
eg (missing part)
$myvar[1]='some value 1';
$myvar[2]=um[1]=10;
$mynum[2]=11;
I have test it in 2 different servers (using local drives, SAN, etc) and I face the same problem.
Thanks in advance for your help