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null is a special unit type in PHP representing a variable with no value. It is the sole possible value of the null data type, indicating the intentional absence of any typed data or object reference. The null constant is case-insensitive, though lowercase null is the accepted standard per PSR-12 guidelines. 
$emptyVariable = null;
$alsoEmpty = NULL; // Valid, but not recommended
A variable is internally evaluated as null under three specific conditions:
  1. It has been explicitly assigned the constant null.
  2. It has been declared but not yet initialized with any value.
  3. It has been explicitly destroyed using the unset() language construct.

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Type Checking and Comparison

Because PHP is a loosely typed language, evaluating null requires strict comparison to avoid unintended type juggling. 
$var = null;

// Strict comparison (Recommended)
// Evaluates to true ONLY if $var is exactly null
$var === null; 

// Built-in type checking function
// Functionally identical to strict comparison
is_null($var); 

// Loose comparison (Not recommended for null checks)
// Evaluates to true for null, false, 0, 0.0, "", and []
$var == null;

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Type Coercion

When a null value is cast or coerced into other scalar or compound types, PHP resolves the absence of value into the empty equivalent of the target type: 
$var = null;

$asBool   = (bool) $var;   // Evaluates to: false
$asInt    = (int) $var;    // Evaluates to: 0
$asFloat  = (float) $var;  // Evaluates to: 0.0
$asString = (string) $var; // Evaluates to: "" (empty string)
$asArray  = (array) $var;  // Evaluates to: [] (empty array)

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Null-Specific Operators

PHP provides specific operators designed to handle null evaluation natively at the syntax level. Null Coalescing Operator (??) Evaluates the left operand. If it is strictly not null and exists, it returns it; otherwise, it returns the right operand. 
$value = $uninitializedVar ?? 'fallback';
Null Coalescing Assignment Operator (??=) Assigns the right operand to the left operand only if the left operand is currently null. 
$var ??= 'assigned_because_null';
Nullsafe Operator (?->) Allows property or method access on an object that might be null. If the object evaluates to null, the entire chain short-circuits and returns null without throwing a fatal error. 
// PHP 8.0+
$result = $object?->method();
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