php - What is the difference between these two array Iterations? -

i'm studying php advanced exam. practice test said first iteration better second. not figure out why. both iterate contents of array fine.

// first one:

 foreach($array $key => &$val) { /* ... */ } 

// second one:

foreach($array $key => $val) { /* ... */ } 

the practice test said first iteration better second.

that isn't best advice. they're different tools different jobs.

the & means treat variable reference opposed copy.

when have variable reference, similar pointer in c. accessing variable lets access memory location of original variable, allowing modify value through different identifier.

// variable. $a = 42;  // reference $a. // & operator returns reference. $ref = &$a;  // assignment of $ref. $ref = "fourty-two";  // $a has changed, through // reference $ref. var_dump($a); // "fourty-two" 

reference example on codepad.

the normal behaviour of foreach make copy available associated block. means free reassign it, , won't affect array member (this won't case variables references, such class instances).

copy example on codepad.

class instance example on codepad.

using reference in foreach has side effects, such dangling $val reference last value iterated over, can modified later accident , affect array.

dangling reference example on codepad.