PHP Pass by reference in foreach

Question

I have this code    a   array   zero   one   two    three     foreach   a as  amp  v        foreach   a as  v      echo  v PHP EOL      Can somebody explain why the output is  zero one two two    From zend certification study guide

User · Answer

This     a   array   zero   one   two    three     foreach   a as  amp  v        foreach   a as  v        echo  v PHP EOL      is the same as   a   array   zero   one   two    three      v    amp  a 3    for   i   0   i  lt  4   i           v    a  i       echo  v PHP EOL       OR   a   array   zero   one   two    three     for   i   0   i  lt  4   i           a 3     a  i       echo  a 3  PHP EOL       OR   a   array   zero   one   two    three      a 3     a 0   echo  a 3  PHP EOL    a 3     a 1    echo  a 3  PHP EOL    a 3     a 2   echo  a 3  PHP EOL    a 3     a 3    echo  a 3  PHP EOL

User · Answer

Because on the second loop   v is still a reference to the last array item  so it s overwritten each time   You can see it like that    a   array   zero   one   two    three     foreach   a as  amp  v        foreach   a as  v      echo  v  -   a 3  PHP EOL      As you can see  the last array item takes the current loop value   zero    one    two   and then it s just  two

User · Answer

I had to spend a few hours to figure out why a 3  is changing on each iteration  This is the explanation at which I arrived    There are two types of variables in PHP   normal variables and reference variables   If we assign a reference of a variable to another variable  the variable becomes a reference variable    for example in   a   array  zero    one    two    three      if we do    v    amp  a 0    the 0th element   a 0   becomes a reference variable    v points towards that variable  therefore  if we make any change to  v  it will be reflected in  a 0  and vice versa    now if we do    v    amp  a 1     a 1  will become a reference variable and  a 0  will become a normal variable  Since no one else is pointing to  a 0  it is converted to a normal variable   PHP is smart enough to make it a normal variable when no one else is pointing towards it   This is what happens in the first loop   foreach   a as  amp  v         After the last iteration  a 3  is a reference variable    Since  v is pointing to  a 3  any change to  v results in a change to  a 3   in the second loop    foreach   a as  v      echo  v  -   a 3  PHP EOL      in each iteration as  v changes   a 3  changes   because  v still points to  a 3    This is the reason why  a 3  changes on each iteration   In the iteration before the last iteration   v is assigned the value  two   Since  v points to  a 3    a 3  now gets the value  two   Keep this in mind    In the last iteration   v  which points to  a 3   now has the value of  two   because  a 3  was set to two in the previous iteration  two is printed  This explains why  two  is repeated when  v is printed in the last iteration

User · Answer

This question has a lot of explanations provided  but no clear examples of how to solve the problem that this behavior causes  In most cases  you ll probably want the following code in your pass by reference foreach  foreach   array as  amp  row           Do stuff      Unset to remove the reference unset  row

User · Answer

First loop   v    a 0    v    a 1    v    a 2    v    a 3     Yes  Current  v    a 3  position   Second loop   a 3     v    a 0   echo  v     same as  a 3  and  a 0      zero   a 3     v    a 1   echo  v     same as  a 3  and  a 1      one   a 3     v    a 2   echo  v     same as  a 3  and  a 2      two   a 3     v    a 3   echo  v     same as  a 3  and  a 3      two    because  a 3  is assigned by before processing

User · Answer

I think this code show the procedure more clear    lt  php   a   array   zero   one   two    three     foreach   a as  amp  v       var dump  a    foreach   a as  v      var dump  a       Result   Take attention on the last two array   array 4       0   gt    string 4   zero     1   gt    string 3   one     2   gt    string 3   two     3   gt     amp string 5   three    array 4       0   gt    string 4   zero     1   gt    string 3   one     2   gt    string 3   two     3   gt     amp string 4   zero    array 4       0   gt    string 4   zero     1   gt    string 3   one     2   gt    string 3   two     3   gt     amp string 3   one    array 4       0   gt    string 4   zero     1   gt    string 3   one     2   gt    string 3   two     3   gt     amp string 3   two    array 4       0   gt    string 4   zero     1   gt    string 3   one     2   gt    string 3   two     3   gt     amp string 3   two

User · Answer

I got here just by accident and the OP s question got my attention  Unfortunately I do not understand any of the explanations from the top  Seems to me like everybody knows it  gets it  accetps it  just cannot explain   Luckily  a pure sentence from PHP documentation on foreach makes this completely clear      Warning  Reference of a  value and the last array element remain even after the foreach loop  It is recommended to destroy it by unset

User · Answer

Because if you create a reference to a variable  all names for that variable  including the original  BECOME REFERENCES

User · Answer

I found this example also tricky  Why that in the 2nd loop at the last iteration nothing happens   v stays  two    is that  v points to  a 3   and vice versa   so it cannot assign value to itself  so it keeps the previous assigned value

[php] PHP Pass by reference in foreach

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