String interpolation in double quotes in PHP - Step-by-Step Execution

Concept Flow - String interpolation in double quotes

Start

↓

Define variable

↓

Use variable inside " "

↓

PHP replaces variable with value

↓

Output final string

↓

End

PHP replaces variables inside double quotes with their values before outputting the string.

Execution Sample

PHP

<?php
$name = "Alice";
echo "Hello, $name!";
?>

This code prints a greeting using the variable $name inside double quotes.

Execution Table

Step	Code Line	Action	Variable Values	Output
1	$name = "Alice";	Assign string 'Alice' to $name	$name = 'Alice'
2	echo "Hello, $name!";	Replace $name with 'Alice' inside string	$name = 'Alice'	Hello, Alice!
3	End	Script ends	$name = 'Alice'

💡 Script ends after echoing the interpolated string.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
$name	undefined	'Alice'	'Alice'	'Alice'

Key Moments - 2 Insights

Why does $name get replaced inside double quotes but not single quotes?

In the execution_table row 2, PHP replaces $name inside double quotes but if single quotes were used, PHP treats it as plain text and does not replace.

What happens if the variable is not defined before interpolation?

If $name was not assigned before echo, PHP would output an empty string or a notice, but here in row 1 $name is defined, so interpolation works.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $name after step 1?

A 'Alice'

B undefined

C "Alice"

D null

💡 Hint

Check the 'Variable Values' column at step 1 in execution_table.

At which step does PHP replace the variable with its value inside the string?

A Step 1

B Step 3

C Step 2

D Before Step 1

💡 Hint

Look at the 'Action' column in execution_table for when replacement happens.

If we used single quotes in echo like echo 'Hello, $name!'; what would be the output?

A Hello, Alice!

B Hello, $name!

C Error

D Hello, !

💡 Hint

Recall that PHP does not interpolate variables inside single quotes, see key_moments explanation.

Concept Snapshot

PHP String Interpolation in Double Quotes:
- Variables inside " " are replaced with their values.
- Use $variable syntax inside double quotes.
- Single quotes ' ' do NOT interpolate variables.
- Useful for embedding variables directly in strings.
- Example: echo "Hello, $name!" outputs Hello, Alice!

Full Transcript

This example shows how PHP replaces variables inside double quotes with their values. First, the variable $name is assigned the string 'Alice'. Then, when echoing "Hello, $name!", PHP replaces $name with 'Alice' and outputs 'Hello, Alice!'. Variables inside single quotes are not replaced. This is a simple way to include variable values inside strings in PHP.