How do I implement a callback in PHP

Question

How are callbacks written in PHP

User · Answer

well    with 5 3 on the horizon  all will be better  because with 5 3  we ll get closures and with them anonymous functions  http   wiki php net rfc closures

User · Answer

Implementation of a callback is done like so     This function uses a callback function   function doIt  callback           data    this is my data        callback  data            This is a sample callback function for doIt     function myCallback  data          print  Data is        data      n            Call doIt   and pass our sample callback function s name   doIt  myCallback      Displays  Data is  this is my data

User · Answer

The manual uses the terms  callback  and  callable  interchangeably  however   callback  traditionally refers to a string or array value that acts like a function pointer  referencing a function or class method for future invocation  This has allowed some elements of functional programming since PHP 4  The flavors are    cb1    someGlobalFunction    cb2     ClassName    someStaticMethod     cb3     object   somePublicMethod        this syntax is callable since PHP 5 2 3 but a string containing it    cannot be called directly  cb2    ClassName  someStaticMethod    cb2       fatal error     legacy syntax for PHP 4  cb3   array  amp  object   somePublicMethod      This is a safe way to use callable values in general   if  is callable  cb2            Autoloading will be invoked to load the class  ClassName  if it s not        yet defined  and PHP will check that the class has a method         someStaticMethod   Note that is callable   will NOT verify that the        method can safely be executed in static context        returnValue   call user func  cb2   arg1   arg2       Modern PHP versions allow the first three formats above to be invoked directly as  cb    call user func and call user func array support all the above   See  http   php net manual en language types callable php  Notes Caveats     If the function class is namespaced  the string must contain the fully-qualified name  E g    Vendor Package Foo    method   call user func does not support passing non-objects by reference  so you can either use call user func array or  in later PHP versions  save the callback to a var and use the direct syntax   cb    Objects with an   invoke   method  including anonymous functions  fall under the category  callable  and can be used the same way  but I personally don t associate these with the legacy  callback  term  The legacy create function   creates a global function and returns its name  It s a wrapper for eval   and anonymous functions should be used instead

User · Answer

I cringe every time I use create function   in php    Parameters are a coma separated string  the whole function body in a string    Argh    I think they could not have made it uglier even if they tried   Unfortunately  it is the only choice when creating a named function is not worth the trouble

User · Answer

One nifty trick that I ve recently found is to use PHP s create function   to create an anonymous lambda function for one-shot use  It s useful for PHP functions like array map    preg replace callback    or usort   that use callbacks for custom processing  It looks pretty much like it does an eval   under the covers  but it s still a nice functional-style way to use PHP

User · Answer

For those who don t care about breaking compatibility with PHP  lt  5 4  I d suggest using type hinting to make a cleaner implementation   function call with hello and append world  callable  callback             No need to check  closure because of the type hint      return  callback   hello     world      function append space   string          return  string          output1   call with hello and append world  function   string     return  string           var dump   output1       string 11   hello world    output2   call with hello and append world   append space     var dump   output2       string 11   hello world    old lambda   create function    string    return  string           output3   call with hello and append world   old lambda    var dump   output3       string 11   hello world

User · Answer

You will want to verify whatever your calling is valid  For example  in the case of a specific function  you will want to check and see if the function exists   function doIt  callback        if function exists  callback              callback          else              some error handling

User · Answer

create function did not work for me inside a class  I had to use call user func    lt  php  class Dispatcher         Added explicit callback declaration      var  callback       public function Dispatcher   callback              this- gt callback    callback             public function asynchronous method             do asynch stuff  like fwrite   then  fire callback         if   isset   this- gt callback                   if  function exists   this- gt callback    call user func   this- gt callback   File done                           Then  to use    lt  php  include once  Dispatcher php     d   new Dispatcher   do callback      d- gt asynchronous method     function do callback   data       print  Data is        data      n       gt     Edit  Added a missing parenthesis   Also  added the callback declaration  I prefer it that way

User · Answer

With PHP 5 3  you can now do this   function doIt  callback     callback       doIt function            this will be done       Finally a nice way to do it  A great addition to PHP  because callbacks are awesome

[php] How do I implement a callback in PHP?

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