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The .= operator is the string concatenation assignment operator in PHP. It appends the string representation of the right-hand operand to the existing string value of the left-hand variable, subsequently assigning the combined result back into the left-hand variable. 
$variable .= expression;
This operation is semantically equivalent to the expanded concatenation and assignment syntax: 
$variable = $variable . expression;

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Technical Mechanics

Operand Requirements
  • Left Operand (L-value): Must be a valid, assignable variable reference. If the variable is uninitialized prior to the operation, PHP implicitly initializes it as an empty string "" before appending the right operand. Note: Relying on uninitialized variables will trigger an E_WARNING in PHP 8.0+.
  • Right Operand: Can be any valid PHP expression that evaluates to a string or a type that can be safely coerced into a string.
Type Coercion Because .= is strictly a string operator, PHP enforces scalar type coercion during execution:
  • Scalars: Integers and floats are automatically cast to their string equivalents. Booleans are cast to "1" (true) or "" (false).
  • Objects: If the right operand is an object, PHP will attempt to call its __toString() magic method. If the method is not implemented, a fatal error is thrown.
  • Arrays: Attempting to append an array using .= will result in an Array to string conversion warning, and the literal string "Array" will be appended.
Memory Allocation At the engine level (Zend Engine), using the .= operator is generally more memory-efficient than the expanded $a = $a . $b syntax. The .= operator allows PHP to mutate the existing string buffer in place (if the variable’s reference count allows it and sufficient memory is allocated), whereas the expanded syntax forces the engine to allocate a new temporary memory block for the concatenated result before reassigning it to the original variable pointer.
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