What s better at freeing memory with PHP unset or var null

Question

I realise the second one avoids the overhead of a function call  update  is actually a language construct   but it would be interesting to know if one is better than the other  I have been using unset   for most of my coding  but I ve recently looked through a few respectable classes found off the net that use  var   null instead   Is there a preferred one  and what is the reasoning

User · Answer

It makes a difference with array elements.

Consider this example

$a = array('test' => 1);
$a['test'] = NULL;
echo "Key test ", array_key_exists('test', $a)? "exists": "does not exist";

Here, the key 'test' still exists. However, in this example

$a = array('test' => 1);
unset($a['test']);
echo "Key test ", array_key_exists('test', $a)? "exists": "does not exist";

the key no longer exists.

User · Answer

unset code if not freeing immediate memory is still very helpful and would be a good practice to do this each time we pass on code steps before we exit a method  take note its not about freeing immediate memory   immediate memory is for CPU  what about secondary memory which is RAM   and this also tackles about preventing memory leaks    please see this link http   www hackingwithphp com 18 1 11 be-wary-of-garbage-collection-part-2  i have been using unset for a long time now   better practice like this in code to instanly unset all variable that have been used already as array     data  tesst        data  test2    asdadsa        nth    and just unset  data   to free all variable usage   please see related topic to unset  How important is it to unset variables in PHP    bug

User · Answer

PHP 7 is already worked on such memory management issues and its reduced up-to minimal usage    lt  php    start   microtime true     for   i   0   i  lt  10000000   i           a    a        a   NULL         elapsed   microtime true  -  start     echo  took  elapsed seconds r n       start   microtime true     for   i   0   i  lt  10000000   i            a    a        unset  a          elapsed   microtime true  -  start     echo  took  elapsed seconds r n      gt    PHP 7 1 Outpu   took 0 16778993606567 seconds  took 0 16630101203918 seconds

User · Answer

It was mentioned in the unset manual s page in 2009   unset   does just what its name says - unset a variable  It does not force immediate memory freeing  PHP s garbage collector will do it when it see fits - by intention as soon  as those CPU cycles aren t needed anyway  or as late as before the script would run out of memory  whatever occurs first  If you are doing  whatever   null  then you are rewriting variable s data  You might get memory freed   shrunk faster  but it may steal CPU cycles from the code that truly needs them sooner  resulting in a longer overall execution time    Since 2013  that unset man page don t include that section anymore  Note that until php5 3  if you have two objects in circular reference  such as in a parent-child relationship  calling unset   on the parent object will not free the memory used for the parent reference in the child object   Nor will the memory be freed when the parent object is garbage-collected    bug 33595   The question  quot difference between unset and   null quot  details some differences    unset  a  also removes  a from the symbol table  for example   a   str repeat  hello world    100   unset  a   var dump  a     Outputs   Notice  Undefined variable  a in xxx NULL   But when  a   null is used    a   str repeat  hello world    100    a   null  var dump  a     Outputs   NULL   It seems that  a   null is a bit faster than its unset   counterpart  updating a symbol table entry appears to be faster than removing it     when you try to use a non-existent  unset  variable  an error will be triggered and the value for the variable expression will be null   Because  what else should PHP do  Every expression needs to result in some value   A variable with null assigned to it is still a perfectly normal variable though

User · Answer

lt  php  start   microtime true   for   i   0   i  lt  10000000   i           a    a        a   NULL     elapsed   microtime true  -  start   echo  took  elapsed seconds r n       start   microtime true   for   i   0   i  lt  10000000   i           a    a       unset  a      elapsed   microtime true  -  start   echo  took  elapsed seconds r n     gt    Per that it seems like    null  is faster   PHP 5 4 results    took 0 88389301300049 seconds took 2 1757180690765 seconds   PHP 5 3 results    took 1 7235369682312 seconds took 2 9490959644318 seconds   PHP 5 2 results    took 3 0069220066071 seconds took 4 7002630233765 seconds   PHP 5 1 results    took 2 6272349357605 seconds took 5 0403649806976 seconds   Things start to look different with PHP 5 0 and 4 4   5 0    took 10 038941144943 seconds took 7 0874409675598 seconds   4 4    took 7 5352551937103 seconds took 6 6245851516724 seconds   Keep in mind microtime true  doesn t work in PHP 4 4 so I had to use the microtime float example given in php net microtime   Example  1

User · Answer

It works in a different way for variables copied by reference    a   5   b    amp  a  unset  b      just say  b should not point to any variable print  a     5   a   5   b    amp  a   b   null     rewrites value of  b  and  a  print  a     nothing  because  a   null

User · Answer

unset is not actually a function  but a language construct  It is no more a function call than a return or an include   Aside from performance issues  using unset makes your code s intent much clearer

User · Answer

For the record  and excluding the time that it takes    lt  php echo   lt hr gt First  lt br gt     x   str repeat  x   80000   echo memory get usage       lt br gt  n         echo memory get peak usage       lt br gt  n    echo   lt hr gt Unset  lt br gt    unset  x    x   str repeat  x   80000   echo memory get usage       lt br gt  n         echo memory get peak usage       lt br gt  n    echo   lt hr gt Null  lt br gt     x null   x   str repeat  x   80000   echo memory get usage       lt br gt  n         echo memory get peak usage       lt br gt  n    echo   lt hr gt function  lt br gt    function test          x   str repeat  x   80000     echo memory get usage       lt br gt  n         echo memory get peak usage       lt br gt  n     echo   lt hr gt Reasign  lt br gt     x   str repeat  x   80000   echo memory get usage       lt br gt  n         echo memory get peak usage       lt br gt  n      It returns  First  438296 438352 Unset  438296 438352 Null  438296 438352 function  438296 438352 Reasign  438296 520216  lt -- double usage    Conclusion  both null and unset free memory as expected  not only at the end of the execution     Also  reassigning a variable holds the value twice at some point  520216 versus 438352

User · Answer

Regarding objects  especially in lazy-load scenario  one should consider garbage collector is running in idle CPU cycles  so presuming you re going into trouble when a lot of objects are loading small time penalty will solve the memory freeing   Use time nanosleep to enable GC to collect memory  Setting variable to null is desirable   Tested on production server  originally the job consumed 50MB and then was halted  After nanosleep was used  14MB was constant memory consumption   One should say this depends on GC behaviour which may change from PHP version to version  But it works on PHP 5 3 fine   eg  this sample  code taken form VirtueMart2 google feed   for  n 0   n lt count  ids    n            unset  product     usefull for arrays      product   null     if   n   50    0                    let GC do the memory job           echo   lt mem gt     memory get usage       lt  mem gt      ids  n           time nanosleep 0  10000000               product    productModel- gt getProductSingle  int  ids  n  true  true  true

User · Answer

By doing an unset   on a variable  you ve essentially marked the variable for  garbage collection   PHP doesn t really have one  but for example s sake  so the memory isn t immediately available  The variable no longer houses the data  but the stack remains at the larger size  Doing the null method drops the data and shrinks the stack memory almost immediately   This has been from personal experience and others as well  See the comments of the unset   function here   I personally use unset   between iterations in a loop so that I don t have to have the delay of the stack being yo-yo d in size  The data is gone  but the footprint remains  On the next iteration  the memory is already being taken by php and thus  quicker to initialize the next variable

User · Answer

I still doubt about this  but I ve tried it at my script and I m using xdebug to know how it will affect my app memory usage  The script is set on my function like this    function gen table data  serv   coorp   type   showSql   FALSE   table    ireg idnts          sql    SELECT COUNT  operator    operator  FROM  table WHERE  serv     coorp        if  showSql     FALSE             sql   mysql query  sql  or die mysql error              data   mysql fetch array  sql           return  data 0         else echo  sql      And I add unset just before the return code and it give me   160200 then I try to change it with  sql   NULL and it give me   160224     But there is something unique on this comparative when I am not using unset   or NULL  xdebug give me 160144 as memory usage  So  I think giving line to use unset   or NULL will add process to your application and it will be better to stay origin with your code and decrease the variable that you are using as effective as you can    Correct me if I m wrong  thanks

User · Answer

Code example from comment echo  quot PHP Version   quot    phpversion     PHP EOL   PHP EOL    start   microtime true   for   i   0   i  lt  10000000   i           a    a        a   NULL     elapsed   microtime true  -  start   echo  quot took  elapsed seconds quot    PHP EOL      start   microtime true   for   i   0   i  lt  10000000   i           a    a       unset  a      elapsed   microtime true  -  start   echo  quot took  elapsed seconds quot    PHP EOL   Running in docker container from image php 7 4-fpm and others   PHP Version  7 4 8 took 0 22569918632507 seconds null took 0 11705803871155 seconds unset took 0 20791196823121 seconds null took 0 11697316169739 seconds unset  PHP Version  7 3 20 took 0 22086310386658 seconds null took 0 11882591247559 seconds unset took 0 21383500099182 seconds null took 0 11916995048523 seconds unset  PHP Version  7 2 32 took 0 24728178977966 seconds null took 0 12719893455505 seconds unset took 0 23839902877808 seconds null took 0 12744522094727 seconds unset  PHP Version  7 1 33 took 0 51380109786987 seconds null took 0 50135898590088 seconds unset took 0 50358104705811 seconds null took 0 50115609169006 seconds unset  PHP Version  7 0 33 took 0 50918698310852 seconds null took 0 50490307807922 seconds unset took 0 50227618217468 seconds null took 0 50514912605286 seconds unset  PHP Version  5 6 40 took 1 0063569545746 seconds null took 1 6303179264069 seconds unset took 1 0689589977264 seconds null took 1 6382601261139 seconds unset  PHP Version  5 4 45 took 1 0791940689087 seconds null took 1 6308979988098 seconds unset took 1 0029168128967 seconds null took 1 6320278644562 seconds unset   But  with other example   lt  php ini set  quot memory limit quot    quot 512M quot     echo  quot PHP Version   quot    phpversion     PHP EOL   PHP EOL    start   microtime true    arr       for   i   0   i  lt  1000000   i           arr      a      arr   null   elapsed   microtime true  -  start   echo  quot took  elapsed seconds quot    PHP EOL      start   microtime true    arr       for   i   0   i  lt  1000000   i           arr      a     unset  arr    elapsed   microtime true  -  start   echo  quot took  elapsed seconds quot    PHP EOL   Results  PHP Version  7 4 8 took 0 053696155548096 seconds took 0 053897857666016 seconds  PHP Version  7 3 20 took 0 054572820663452 seconds took 0 054342031478882 seconds  PHP Version  7 2 32 took 0 05678391456604 seconds took 0 057311058044434 seconds   PHP Version  7 1 33 took 0 097366094589233 seconds took 0 073100090026855 seconds  PHP Version  7 0 33 took 0 076443910598755 seconds took 0 077098846435547 seconds  PHP Version  7 0 33 took 0 075634002685547 seconds took 0 075317859649658 seconds  PHP Version  5 6 40 took 0 29681086540222 seconds took 0 28199100494385 seconds  PHP Version  5 4 45 took 0 30513095855713 seconds took 0 29265689849854 seconds

User · Answer

I created a new performance test for unset and  null  because as mentioned in the comments the here written has an error  the recreating of the elements    I used arrays  as you see it didn t matter now    lt  php  arr1   array     arr2   array    for   i   0   i  lt  10000000   i           arr1  i     a        arr2  i     a       start   microtime true   for   i   0   i  lt  10000000   i           arr1  i    null     elapsed   microtime true  -  start   echo  took     elapsed   seconds lt br gt      start   microtime true   for   i   0   i  lt  10000000   i          unset  arr2  i       elapsed   microtime true  -  start   echo  took     elapsed   seconds lt br gt      But i can only test it on an PHP 5 5 9 server  here the results   - took 4 4571571350098 seconds  - took 4 4425978660583 seconds  I prefer unset for readability reasons

[php] What's better at freeing memory with PHP: unset() or $var = null

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