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The = operator in PHP is the basic assignment operator. It evaluates the expression on its right-hand side (RHS) and writes the resulting value into the memory location referenced by the operand on its left-hand side (LHS). 
$lvalue = expression;

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Technical Characteristics

L-values and R-values The LHS operand must be a valid l-value (a variable, array index, or object property). The RHS operand can be any valid r-value, which includes literals, variables, function return values, or complex expressions. Return Value In PHP, assignment is an expression, not just a statement. The = operator returns the value that was assigned to the LHS. This allows the assignment operation to be embedded within larger expressions. 
// The expression ($b = 5) assigns 5 to $b and evaluates to 5.
// $a is then assigned the result of 5 + 10.
$a = ($b = 5) + 10;
Associativity The = operator has right-to-left associativity. When multiple assignment operators are chained, the PHP engine evaluates the rightmost assignment first and propagates the return value to the left. 
// Evaluated as: $x = ($y = ($z = 0));
$x = $y = $z = 0;

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Memory and Evaluation Behavior

The behavior of the = operator depends on the data type of the RHS expression: 1. Assignment by Value (Scalars and Arrays) For scalar types (integers, floats, strings, booleans) and arrays, the = operator performs assignment by value. PHP creates a copy of the RHS value and assigns it to the LHS. Subsequent modifications to the LHS variable do not affect the RHS variable. (Note: Internally, PHP optimizes array assignment using Copy-on-Write (CoW), deferring actual memory duplication until one of the variables is modified). 
$a = [1, 2, 3];
$b = $a; // $b receives a copy of the array
2. Object Identifier Assignment When the RHS is an object, the = operator does not copy the object itself. Instead, it copies the object identifier. Both the LHS and RHS variables will hold distinct copies of the identifier that point to the exact same object instance in memory. 
$objA = new stdClass();
$objB = $objA; // Both variables point to the same instance
3. Reference Assignment (=&) To bypass standard assignment behavior and bind two variables to the exact same memory address (zval), the = operator must be combined with the reference operator (&). This creates an alias, meaning neither variable owns the value exclusively. 
$x = 100;
$y =& $x; // $y is now a reference to $x
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