What is the limit in size of a Php variable when storing string?

Question

This is the case: I have a 2Gb dump file called myDB.sql. It is a dump file that deletes a existing database and creates a new one, with views and triggers. So I have the string myDB_OLD spread for many lines of the code. I would like to change these strings occurrences to myDB_NEW. I could do this easelly using notePad++. But notepad does not opens a 2Gb file. What I did is a PHP code that reads line by line and find and replace the string I want.

This is the code:

$myfile2 = fopen("myDB.sql", "r") or die("Unable to open file!");//Reads the file
while (!feof($myfile2)) {//Pass trough each line
    $str=fgets($myfile2);//Store the line inside a variable
    if (preg_match("/myDB_OLD/",$str)) {//If the string I want to change exists - replace it and conacatenate
        $newStr .= str_replace("myDB_OLD","myDB_NEW",$str);
    } 
    else {//If not concatenate it
        $newStr .=$str;
    }
}//End of while
fclose($myfile2);
//Save the  newStr in a new file named 
$myfile = fopen("newDB.sql", "w") or die("Unable to open file!");
fwrite($myfile,  $newStr);
echo "finished";

This code retrieve each line of the file changes the string, concatenate in variable and creates a new file. It should works but it is not. I dont know why. I am using xdebug to figure out what is the issue, but no luck.

So I change the approach. Instead of save each line in a variable, I save it directly in a file and that works good.

This is the new code:

$myfile = fopen("newDB.sql", "w") or die("Unable to open file!");//Creates a new file "newDB.sql"

$myfile2 = fopen("myDB.sql", "r") or die("Unable to open file!");//Reads the file
while (!feof($myfile2)) {//Pass trough each line
        $str=fgets($myfile2);//Store the line inside a variable
        if (preg_match("/myDB/",$str)) {//If the string I want to change exists - replace it . (Do not concatenate)
            $strRep=str_replace("myDB","myDB_NEW",$str);
        }
        else {
            $strRep =$str;
        }

        fwrite($myfile,  $strRep);// Add the new line to the file "newDB.sql"
}//End of while
fclose($myfile);
fclose($myfile2);

echo "finished";

Ok, I solved my issue but it raises a thought. What is the issue of the first code? I think the issue is the amount of information to be stored in a PHP variable, 2Gb. So, is there a limit in size to a PHP variable to stores value, in this case, a string ? If yes how can I check or change it? Any php.ini variable?

What error does it give? And what is the memory limit set in your php.ini file? — Mark Baker, Nov 23 '15 at 15:29
And as you can import a MySQL dump to any database name, why do you even need to do this in the first place? — Mark Baker, Nov 23 '15 at 15:30
For the first question. There is no error. Xdebugt do not throw any error. It only stops to work inside the loop. For the memory limit I could check it if I new this is the case this is why I didn check it as my first approach before to rewire the code. For the second question. It was a specift situation and I dont know if I change the database name it will change all occurences inside the dump file like creates triggers and etc. I can test it. Do you know it? — IgorAlves, Nov 23 '15 at 15:48

score 10 · Accepted Answer · edited Feb 13 '20 at 09:43

10

So, is there a limit in size to a Php variable to stores value, in this case, a string ?

Yes. A string can be as large as up to 2GB (2147483647 bytes maximum). You can override this limit by increasing memory_limit directive in php.ini.

From php7 there's not that limitation in a 64 bit system:

Support for strings with length >= 2^31 bytes in 64 bit builds.

edited Feb 13 '20 at 09:43

Jaume Jiménez

33
6

answered Nov 23 '15 at 15:35

Federkun

36,084
8
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2

Good one. Hadn't even thought of that. But, as mentioned above - a PHP application using over 2GB is probably worth refactoring :) – Rob Nov 23 '15 at 15:39
What are you talking about @Rob Quist??? a PHP application using over 2Gb? What??? He's got 2gb of DATA. not 2gb worth of PHP stuff... He's writing a php code to copy over a 2gb database... I'm working with a 10Gb database right now, and worked with databases of up to 200gb in the past... Why are you saying I should do some refactoring? I got a ton of data, and thats it. My php code isn't at fault. Nothing is at fault. There is just a lot of users and a lot of manual and automatic data entry... Every single line in that database is useful and necessary... – Mathieu Turcotte Feb 22 '17 at 19:09
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I mean if you're code is using 2GB RAM at a single moment, its worth refactoring. Wasn't talking about data, obviously. – Rob Feb 23 '17 at 10:01

Hasse Björk · Answer 2 · 2015-11-23T16:03:29.827

4

The maximum number of bytes a script is allowed to allocate is set in php.ini. Look for memory_limit. This is 16Mb after PHP 5.2.0 as a default!

This can be changed by doing a:

ini_set('memory_limit','24M');

The maximum size of a string is 2 GB in PHP, probably because of adressing limitations, if allowed by memory_limit.

edited Nov 23 '15 at 16:03

answered Nov 23 '15 at 15:31

Hasse Björk

1,431
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Indeed. Allthough you're probably doing something wrong if you need huge amounts there. The line-by-line method you've done there is a proper solution. – Rob Nov 23 '15 at 15:32
3

Additionally, this limit can be increased using `set_memory_limit()`. – Peter Nov 23 '15 at 15:32
3

@RobQuist You are quite right, though sometimes when parsing xls or pdf files the size can grow beyond 16M – Peter Nov 23 '15 at 15:34
1

@Peter, sorry I did not find `set_memory_limit`, but rather `ini_set('memory_limit','16M');` – Hasse Björk Nov 23 '15 at 15:42

What is the limit in size of a Php variable when storing string?

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