Assignment Operators
The basic assignment operator is "=". Your first inclination might
be to think of this as "equal to". Don't. It really means that the
left operand gets set to the value of the expression on the
right (that is, "gets set to").
The value of an assignment expression is the value assigned. That
is, the value of "$a = 3
" is 3. This allows you to do some tricky
things:
In addition to the basic assignment operator, there are "combined
operators" for all of the binary
arithmetic, array union and string operators that allow you to use a value in an
expression and then set its value to the result of that expression. For
example:
Note that the assignment copies the original variable to the new
one (assignment by value), so changes to one will not affect the
other. This may also have relevance if you need to copy something
like a large array inside a tight loop.
An exception to the usual assignment by value behaviour within PHP occurs
with objects, which are assigned by reference.
Objects may be explicitly copied via the clone keyword.
Assignment by Reference
Assignment by reference is also supported, using the
"$var = &$othervar;" syntax.
Assignment by reference means that both variables end up pointing at the
same data, and nothing is copied anywhere.
Example #1 Assigning by reference
<?php
$a = 3;
$b = &$a; // $b is a reference to $a
print "$a\n"; // prints 3
print "$b\n"; // prints 3
$a = 4; // change $a
print "$a\n"; // prints 4
print "$b\n"; // prints 4 as well, since $b is a reference to $a, which has
// been changed
?>
The new
operator returns a reference automatically, as such assigning the result of
new by reference is an error.
More information on references and their potential uses can be found in
the References Explained
section of the manual.
Arithmetic Assignment Operators
Bitwise Assignment Operators
Other Assignment Operators
Peter, Moscow ¶13 years ago
Using $text .= "additional text"; instead of $text = $text ."additional text"; can seriously enhance performance due to memory allocation efficiency.
I reduced execution time from 5 sec to .5 sec (10 times) by simply switching to the first pattern for a loop with 900 iterations over a string $text that reaches 800K by the end.
Robert Schneider ¶9 years ago
Be aware of assignments with conditionals. The assignment operator is stronger as 'and', 'or' and 'xor'.
<?php
$x = true and false; $y = (true and false); ?>
Hayley Watson ¶16 years ago
bradlis7 at bradlis7 dot com's description is a bit confusing. Here it is rephrased.
<?php
$a = 'a';
$b = 'b';
$a .= $b .= "foo";
echo $a,"\n",$b;?>
outputs
abfoo
bfoo
Because the assignment operators are right-associative and evaluate to the result of the assignment
<?php
$a .= $b .= "foo";
?>
is equivalent to
<?php
$a .= ($b .= "foo");
?>
and therefore
<?php
$b .= "foo";
$a .= $b;
?>
asc at putc dot de ¶9 years ago
PHP uses a temporary variable for combined assign-operators (unlike JavaScript), therefore the left-hand-side (target) gets evaluated last.
Input:
$a += $b + $c;
Meaning:
$a = ($b + $c) + $a;
Not:
$a = $a + ($b + $c);
This can be important if the target gets modified inside the expression.
$a = 0;
$a += (++$a) + (++$a); // yields 5 (instead of 4)