In PHP what is a closure and why does it use the use identifier

Question

I m checking out some PHP 5 3 0 features and ran across some code on the site that looks quite funny   public function getTotal  tax         total   0 00        callback              This line here             function   quantity   product  use   tax   amp  total                         pricePerItem   constant   CLASS        PRICE                     strtoupper  product                 total      pricePerItem    quantity      tax   1 0                   array walk  this- gt products   callback       return round  total  2       as one of the examples on anonymous functions   Does anybody know about this  Any documentation  And it looks evil  should it ever be used

User · Answer

A simpler answer   function   quantity  use   tax   amp  total            The closure is a function assigned to a variable  so you can pass it around A closure is a separate namespace  normally  you can not access variables defined outside of this namespace  There comes the use keyword  use allows you to access  use  the succeeding variables inside the closure  use is early binding  That means the variable values are COPIED upon DEFINING the closure  So modifying  tax inside the closure has no external effect  unless it is a pointer  like an object is  You can pass in variables as pointers like in case of  amp  total  This way  modifying the value of  total DOES HAVE an external effect  the original variable s value changes  Variables defined inside the closure are not accessible from outside the closure either  Closures and functions have the same speed  Yes  you can use them all over your scripts    As  Mytskine pointed out probably the best in-depth explanation is the RFC for closures   Upvote him for this

User · Answer

Zupa did a great job explaining closures with  use  and the difference between EarlyBinding and Referencing the variables that are  used     So I made a code example with early binding of a variable    copying       lt  php   a   1   b   2    closureExampleEarlyBinding   function   use   a   b        a         b        echo  Inside   closureExampleEarlyBinding     a      a   lt br   gt        echo  Inside   closureExampleEarlyBinding     b      b   lt br   gt            echo  Before executing   closureExampleEarlyBinding     a      a   lt br   gt    echo  Before executing   closureExampleEarlyBinding     b      b   lt br   gt        closureExampleEarlyBinding     echo  After executing   closureExampleEarlyBinding     a      a   lt br   gt    echo  After executing   closureExampleEarlyBinding     b      b   lt br   gt        this will output  Before executing  closureExampleEarlyBinding    a   1 Before executing  closureExampleEarlyBinding    b   2 Inside  closureExampleEarlyBinding    a   2 Inside  closureExampleEarlyBinding    b   3 After executing  closureExampleEarlyBinding    a   1 After executing  closureExampleEarlyBinding    b   2       gt    Example with referencing a variable  notice the   amp   character before variable     lt  php   a   1   b   2    closureExampleReferencing   function   use   amp  a   amp  b        a         b        echo  Inside   closureExampleReferencing     a      a   lt br   gt        echo  Inside   closureExampleReferencing     b      b   lt br   gt         echo  Before executing   closureExampleReferencing     a      a   lt br   gt    echo  Before executing   closureExampleReferencing     b      b   lt br   gt         closureExampleReferencing     echo  After executing   closureExampleReferencing     a      a   lt br   gt    echo  After executing   closureExampleReferencing     b      b   lt br   gt            this will output  Before executing  closureExampleReferencing    a   1 Before executing  closureExampleReferencing    b   2 Inside  closureExampleReferencing    a   2 Inside  closureExampleReferencing    b   3 After executing  closureExampleReferencing    a   2 After executing  closureExampleReferencing    b   3       gt

User · Answer

closures are beautiful  they solve a lot of problems that come with anonymous functions  and make really elegant code possible  at least as long as we talk about php    javascript programmers use closures all the time  sometimes even without knowing it  because bound variables aren t explicitly defined - that s what  use  is for in php   there are better real-world examples than the above one  lets say you have to sort an multidimensional array by a sub-value  but the key changes    lt  php     function generateComparisonFunctionForKey  key            return function   left   right  use   key                if   left  key      right  key                   return 0              else                 return   left  key   lt   right  key     -1   1                         myArray   array          array  name    gt   Alex    age    gt  70           array  name    gt   Enrico    age    gt  25               sortByName   generateComparisonFunctionForKey  name         sortByAge    generateComparisonFunctionForKey  age         usort  myArray   sortByName        usort  myArray   sortByAge     gt    warning  untested code  i don t have php5 3 installed atm   but it should look like something like that   there s one downside  a lot of php developers may be a bit helpless if you confront them with closures   to understand the nice-ty of closures more  i ll give you another example - this time in javascript  one of the problems is the scoping and the browser inherent asynchronity  especially  if it comes to window setTimeout     or -interval   so  you pass a function to setTimeout  but you can t really give any parameters  because providing parameters executes the code   function getFunctionTextInASecond value        return function              document getElementsByName  body   0  innerHTML   value      value  is the bound variable           var textToDisplay   prompt  text to show in a second    foo bar        this returns a function that sets the bodys innerHTML to the prompted value var myFunction   getFunctionTextInASecond textToDisplay    window setTimeout myFunction  1000     myFunction returns a function with a kind-of predefined parameter   to be honest  i like php a lot more since 5 3 and anonymous functions closures  namespaces may be more important  but they re a lot less sexy

User · Answer

This is how PHP expresses a closure   This is not evil at all and in fact it is quite powerful and useful     Basically what this means is that you are allowing the anonymous function to  capture  local variables  in this case   tax and a reference to  total  outside of it scope and preserve their values  or in the case of  total the reference to  total itself  as state within the anonymous function itself

User · Answer

The function    use       is like closure for PHP   Without use  function cannot access parent scope variable   s    hello    f   function          echo  s       f       Notice  Undefined variable  s    s    hello    f   function    use   s        echo  s       f       hello   The use variable s value is from when the function is defined  not when called   s    hello    f   function    use   s        echo  s       s    how are you     f       hello   use variable by-reference with  amp    s    hello    f   function    use   amp  s        echo  s       s    how are you     f       how are you

User · Answer

Until very recent years  PHP has defined its AST and PHP interpreter has isolated the parser from the evaluation part   During the time when the closure is introduced  PHP s parser is highly coupled with the evaluation   Therefore when the closure was firstly introduced to PHP  the interpreter has no method to know which which variables will be used in the closure  because it is not parsed yet   So user has to pleased the zend engine by explicit import  doing the homework that zend should do   This is the so-called simple way in PHP

[php] In PHP, what is a closure and why does it use the "use" identifier?

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