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

Concept Flow - Null coalescing in conditions

Start

↓

Check if variable is set and not null

↓

Use variable

↓

Continue with chosen value

↓

End

The program checks if a variable exists and is not null; if yes, it uses that variable, otherwise it uses a default value.

Execution Sample

PHP

<?php
$value = $input ?? 'default';
if ($value) {
    echo "Value is: $value";
} else {
    echo "No value";
}
?>

This code uses null coalescing to assign $value and then checks if $value is truthy to print a message.

Execution Table

Step	Variable $input	Expression $input ?? 'default'	Condition if($value)	Branch Taken	Output
1	null	'default'	if('default')	True branch	Value is: default
2	'Hello'	'Hello'	if('Hello')	True branch	Value is: Hello
3	'' (empty string)	''	if('')	False branch	No value
4	0	0	if(0)	False branch	No value
5	false	false	if(false)	False branch	No value
6	unset (undefined)	'default'	if('default')	True branch	Value is: default

💡 Execution stops after printing output for each test case.

Variable Tracker

Variable	Start	Test 1	Test 2	Test 3	Test 4	Test 5	Test 6
$input	undefined	null	'Hello'	''	0	false	undefined
$value	undefined	'default'	'Hello'	''	0	false	'default'

Key Moments - 3 Insights

Why does the condition fail when $value is an empty string or 0?

Because in PHP, empty string and 0 are considered false in conditions, so the else branch runs as shown in rows 3 and 4 of the execution_table.

What happens if $input is not set at all?

The null coalescing operator uses the default value, so $value becomes 'default' and the if condition is true, as shown in row 6.

Does null coalescing check only for null or also for undefined variables?

It checks both: if the variable is not set or is null, it uses the default value, demonstrated in rows 1 and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $value at Step 2 when $input is 'Hello'?

A 'default'

B 'Hello'

C null

D '' (empty string)

💡 Hint

Check the 'Expression $input ?? 'default'' column at Step 2 in the execution_table.

At which step does the condition if($value) evaluate to false because $value is 0?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look at the 'Condition if($value)' column and find where the value is 0.

If $input was set to false, what output would the program produce according to the execution_table?

A No value

B Value is: false

C Value is: default

D Error

💡 Hint

Check Step 5 output in the execution_table where $input is false.

Concept Snapshot

Null coalescing operator (??) returns the left value if set and not null; otherwise, it returns the right value.
Used in conditions to provide defaults.
Example: $value = $input ?? 'default';
If $input is null or unset, $value is 'default'.
Conditions treat '', 0, false as false even if not null.

Full Transcript

This visual execution shows how PHP's null coalescing operator works in conditions. It checks if a variable is set and not null; if yes, it uses that variable; otherwise, it uses a default value. The execution table traces different values of the input variable and how the condition if($value) behaves. For example, when input is null or unset, the default is used. When input is an empty string or zero, the condition evaluates to false because PHP treats these as false in conditions. The variable tracker shows how $input and $value change in each test. Key moments clarify common confusions about falsey values and unset variables. The quiz tests understanding of these behaviors by referencing the execution table and variable tracker.