isset() checks if a variable is declared and not null, returning true for empty strings, 0, '0', false, and empty arrays; use it to confirm a variable exists and has been set, such as verifying form inputs like $_POST['email']. 2. empty() determines if a value is "empty" in a user-logic sense, returning true for '', 0, '0', null, false, [], and 0.0, and is safe for undefined variables; use it when treating '0' or 0 as invalid or not provided, such as in optional user input fields. 3. is_null() strictly returns true only if a variable exists and is explicitly null, but it misleadingly returns true for undefined variables due to PHP's behavior; use it only when the variable is guaranteed to exist and you need to distinguish null from other falsy values. To safely check for undefined or null, combine with isset(): if (!isset($var) || $var === null), and for strict null checks, use if (isset($var) && $var === null).

When validating variables in PHP, isset(), empty(), and is_null() are commonly used—but they behave differently and are often misunderstood. Choosing the wrong one can lead to unexpected results, especially when dealing with user input, form data, or API responses. Let’s break down the nuances so you know exactly when to use each.

1. What isset() Really Checks

isset() determines whether a variable is declared and not null.

$var = null;
echo isset($var) ? 'Yes' : 'No'; // Outputs: No

$var = '';
echo isset($var) ? 'Yes' : 'No'; // Outputs: Yes

• Returns false if:
  • The variable doesn’t exist.
  • The variable exists but is assigned null.
• Returns true for empty strings, 0, '0', false, arrays with no elements—anything except null or undefined.

? Use isset() when you want to know if a variable has been set and has a meaningful value (even if that value is logically "empty").

Common use case: Checking if $_POST['email'] was sent in a form.

2. How empty() Interprets "Empty"

empty() returns true if a variable is considered "empty" in a loose, user-logic sense.

These values all return true for empty():

• '' (empty string)
• 0
• '0'
• null
• false
• [] (empty array)
• 0.0

var_dump(empty('0')); // true — often surprising!
var_dump(empty([]));  // true
var_dump(empty(null)); // true

? Use empty() when you're checking user input where values like '0' or 0 should be treated as "not provided" or "invalid."

Note: empty() does not generate a warning if the variable doesn't exist—unlike using the variable directly. So it's safe for undefined vars.

3. When to Use is_null()

is_null() is strict: it only returns true if the variable exists and is explicitly null.

$var;
var_dump(is_null($var)); // true (implicit null)

$var = null;
var_dump(is_null($var)); // true

$var = '';
var_dump(is_null($var)); // false

But:

var_dump(is_null($undefined)); // true — even if undefined!

Wait—yes, is_null() returns true for undefined variables because PHP passes null by default when a variable doesn’t exist.

?? So: is_null() is not safe for checking undefined variables unless you know they’re already declared.

? Use is_null() only when you’re certain the variable exists and you need to distinguish null from other falsy values.

Key Differences at a Glance

*is_null($undefined) returns true, but that’s misleading—because the variable doesn’t exist.

Best Practices & Recommendations

• ? Use isset() to check if a variable (like $_GET['id']) was provided.
• ? Use empty() for user input where '0' or 0 should be treated as missing (e.g., optional fields).
• ? Avoid is_null() unless you’re in a context where the variable is guaranteed to exist.
• ? For strict checks, combine tools:

  if (isset($var) && $var === null) { /* explicitly null */ }

• ?? To safely check undefined or null:

  if (!isset($var) || $var === null) { /* not set or null */ }

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  Basically, the choice depends on what you’re trying to validate: existence, usability, or strict nullity. Mixing them up leads to bugs—especially when '0' is valid data (like a zero rating). Know the difference, and test edge cases.

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