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.