Null coalescing operator in PHP - Step-by-Step Execution

Concept Flow - Null coalescing operator

Check if variable isset and not null

Yes↓

Use variable value

↓

END

No↓

Use fallback value

↓

END

The null coalescing operator checks if a variable exists and is not null; if so, it uses that value, otherwise it uses a fallback.

Execution Sample

PHP

<?php
$username = $_GET['user'] ?? 'guest';
echo $username;
?>

This code sets $username to the 'user' parameter from the URL if it exists; otherwise, it uses 'guest'.

Execution Table

Step	Expression	Check	Result	Value assigned to $username	Output
1	$_GET['user'] ?? 'guest'	Is $_GET['user'] set and not null?	No	'guest'	guest
2	echo $username;	Output the value of $username	N/A	N/A	guest
3	End of script	N/A	N/A	N/A	N/A

💡 Execution stops after echoing the value of $username.

Variable Tracker

Variable	Start	After Step 1	Final
$username	undefined	'guest'	'guest'

Key Moments - 2 Insights

Why does the operator check if the variable is set and not just if it is null?

Because the null coalescing operator (??) first checks if the variable exists (is set) and is not null before using it. If the variable is not set at all, it uses the fallback. This is shown in execution_table step 1 where $_GET['user'] is not set, so 'guest' is used.

What happens if $_GET['user'] is set but its value is null?

The operator treats null as if the variable is not set and uses the fallback value. This means it will assign 'guest' to $username, as the operator checks for both existence and non-null value.

Visual Quiz - 3 Questions

Look at the execution table, what value is assigned to $username at step 1?

A null

B 'guest'

C undefined

D 'user'

💡 Hint

Refer to the 'Value assigned to $username' column in step 1 of the execution_table.

At which step does the script output the value of $username?

A Step 1

B Step 3

C Step 2

D No output step

💡 Hint

Check the 'Output' column in the execution_table for the echo statement.

If $_GET['user'] was set to 'alice', how would the value of $username change at step 1?

A 'alice'

B 'guest'

C null

D undefined

💡 Hint

The null coalescing operator uses the variable's value if it is set and not null, see concept_flow.

Concept Snapshot

Null coalescing operator (??) syntax:
$var = $possibleNull ?? $fallback;
It returns $possibleNull if set and not null,
otherwise returns $fallback.
Useful for default values when variables may be missing or null.

Full Transcript

The null coalescing operator in PHP checks if a variable exists and is not null. If it is, it uses that variable's value; if not, it uses a fallback value. For example, $username = $_GET['user'] ?? 'guest'; means if the 'user' parameter exists in the URL and is not null, $username gets that value; otherwise, it gets 'guest'. The execution table shows step 1 checking if $_GET['user'] is set and not null. Since it is not set, the fallback 'guest' is assigned. Step 2 outputs $username, which is 'guest'. This operator helps avoid errors from undefined variables and provides a simple way to set default values.

Practice

1. What does the null coalescing operator ?? do in PHP?

easy

A. Returns the first operand if it is not null, otherwise returns the second operand.

B. Returns true if both operands are null.

C. Checks if a variable is set and throws an error if not.

D. Assigns null to a variable if it is not already set.

Solution

1. Step 1: Understand the operator's purpose

   The null coalescing operator ?? checks if the left value is null.

2. Step 2: Determine the returned value

   If the left value is not null, it returns that; otherwise, it returns the right value.

3. Final Answer:

   Returns the first operand if it is not null, otherwise returns the second operand. -> Option A

4. Quick Check:

   Null coalescing operator returns first non-null value [OK]

Hint: Remember: returns first non-null value [OK]

Common Mistakes:

• Thinking it returns true/false
• Confusing it with the ternary operator
• Assuming it throws errors if variable is unset

2. Which of the following is the correct syntax to use the null coalescing operator in PHP?

easy

A. $value = $a ??= $b;

B. $value = $a ?: $b;

C. $value = $a ? $b;

D. $value = $a ?? $b;

Solution

1. Step 1: Identify the null coalescing operator syntax

   The operator is written as ?? between two expressions.

2. Step 2: Check each option

   $value = $a ?? $b; uses $a ?? $b which is correct syntax. $value = $a ?: $b; uses the ternary shorthand, not null coalescing. $value = $a ? $b; is incomplete syntax. $value = $a ??= $b; uses ??= which is a different operator (null coalescing assignment).

3. Final Answer:

   $value = $a ?? $b; -> Option D

4. Quick Check:

   Correct null coalescing syntax uses ?? between values [OK]

Hint: Use ?? between two values for null coalescing [OK]

Common Mistakes:

• Confusing ?? with ?: operator
• Using incomplete or invalid syntax
• Mixing null coalescing with assignment operator

3. What will be the output of the following PHP code?

$a = null;
$b = 5;
$c = $a ?? $b;
echo $c;

medium

A. null

B. 0

C. 5

D. Error

Solution

1. Step 1: Evaluate the variables

   $a is null, $b is 5.

2. Step 2: Apply null coalescing operator

   $c = $a ?? $b means if $a is not null use $a, else use $b. Since $a is null, $c becomes 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Null coalescing returns 5 when left is null [OK]

Hint: If left is null, output right value [OK]

Common Mistakes:

• Assuming output is null
• Confusing with ternary operator behavior
• Expecting error due to null variable

4. Identify the error in the following PHP code using the null coalescing operator:

$value = ?? 10;

medium

A. Missing left operand before ?? operator.

B. Missing semicolon at the end.

C. Using wrong operator, should be ?: instead of ??.

D. No error, code is correct.

Solution

1. Step 1: Analyze the operator usage

   The null coalescing operator requires a left and right operand, like $a ?? $b.

2. Step 2: Identify the error in the code

   The code has ?? 10 without a left operand before ??, which is invalid syntax.

3. Final Answer:

   Missing left operand before ?? operator. -> Option A

4. Quick Check:

   Null coalescing needs both operands [OK]

Hint: Both sides needed for ?? operator [OK]

Common Mistakes:

• Leaving out left operand
• Confusing ?? with ternary operator
• Ignoring syntax errors

5. Given the array $data = ['name' => null, 'age' => 25];, which code snippet correctly assigns $result to the value of 'name' if it exists and is not null, otherwise to 'Unknown'?

hard

A. $result = $data['name'] ?? 'Unknown';

B. $result = isset($data['name']) ? $data['name'] : 'Unknown';

C. $result = $data['name'] ?: 'Unknown';

D. $result = $data['name'] ??= 'Unknown';

Solution

1. Step 1: Understand the difference between ?? and isset()

   The null coalescing operator returns the right value if the left is null or not set. However, if the key exists with a null value, $data['name'] ?? 'Unknown' returns 'Unknown'.

2. Step 2: Check each option's behavior

   $result = $data['name'] ?? 'Unknown'; returns 'Unknown' because 'name' is null. $result = isset($data['name']) ? $data['name'] : 'Unknown'; uses isset() which returns false if null, so it returns 'Unknown'. $result = $data['name'] ?: 'Unknown'; uses ?: which treats null as false, so returns 'Unknown'. $result = $data['name'] ??= 'Unknown'; uses ??= which assigns 'Unknown' to $data['name'] if null or unset, then returns it.

3. Step 3: Choose the best option for the requirement

   The question asks to assign $result to 'name' if it exists and is not null, else 'Unknown'. Since 'name' exists but is null, only $result = isset($data['name']) ? $data['name'] : 'Unknown'; correctly checks existence and returns 'Unknown'. $result = $data['name'] ?? 'Unknown'; and C treat null as missing, $result = $data['name'] ??= 'Unknown'; modifies the array which is not asked.

4. Final Answer:

   $result = isset($data['name']) ? $data['name'] : 'Unknown'; -> Option B

5. Quick Check:

   Use isset() to check existence and null separately [OK]

Hint: Use isset() to check key exists and not null [OK]

Common Mistakes:

• Assuming ?? checks key existence only
• Using ??= which changes original array
• Confusing ?: with ?? operator