Variable naming rules in PHP - Step-by-Step Execution

Concept Flow - Variable naming rules

Start

↓

Check first character

↓

Check rest characters

↓

Valid variable name

↓

Use variable

↓

Error: invalid characters

↓

Error: invalid first character

Variable names must start with a letter or underscore, followed by letters, digits, or underscores only.

Execution Sample

PHP

<?php
$var1 = 10;
$_var2 = 20;
$3var = 30; // invalid
$var_3 = 40;
?>

This code shows valid and invalid variable names in PHP.

Execution Table

Step	Variable Name	Check First Character	Check Remaining Characters	Result
1	$var1	v (letter) - valid	a,r,1 (letters/digit) - valid	Valid variable name
2	$_var2	_ (underscore) - valid	v,a,r,2 (letters/digit) - valid	Valid variable name
3	$3var	3 (digit) - invalid	N/A	Error: invalid first character
4	$var_3	v (letter) - valid	a,r,_,3 (letters/digit/underscore) - valid	Valid variable name

💡 Step 3 stops due to invalid first character '3' in variable name.

Variable Tracker

Variable Name	Start	After Check First Character	After Check Remaining Characters	Final Result
$var1	Not checked	Valid	Valid	Valid variable name
$_var2	Not checked	Valid	Valid	Valid variable name
$3var	Not checked	Invalid	N/A	Error: invalid first character
$var_3	Not checked	Valid	Valid	Valid variable name

Key Moments - 3 Insights

Why can't a variable name start with a digit like $3var?

Because PHP requires the first character to be a letter or underscore, as shown in execution_table step 3 where '3' is invalid.

Are digits allowed anywhere in the variable name?

Yes, digits can appear after the first character, as shown in $var1 and $var_3 in execution_table steps 1 and 4.

Is underscore allowed at the start of a variable name?

Yes, underscore is allowed as the first character, as shown in $_var2 in execution_table step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the result of checking the variable name $3var at step 3?

A Error: invalid first character

B Valid variable name

C Error: invalid characters in the middle

D Valid but discouraged

💡 Hint

Refer to execution_table row 3 under 'Result' column.

At which step does the variable name start with an underscore?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check 'Check First Character' column in execution_table.

If we change $var_3 to $var-3, what would happen in the checks?

A Error: invalid first character

B Error: invalid characters in the middle

C It would be valid

D No error, just a warning

💡 Hint

Hyphen '-' is not allowed in variable names, see rules in concept_flow.

Concept Snapshot

PHP variable names must start with a letter or underscore (_).
After the first character, letters, digits, and underscores are allowed.
Variable names cannot start with digits or contain special characters.
Examples: $var1, $_var2, $var_3 are valid; $3var is invalid.

Full Transcript

In PHP, variable names must start with a letter or underscore. The rest of the name can include letters, digits, or underscores. For example, $var1 and $_var2 are valid variable names. However, $3var is invalid because it starts with a digit. This rule helps PHP understand variable names correctly and avoid errors. When checking variable names, first the initial character is verified, then the rest are checked for allowed characters. If any character is invalid, PHP will raise an error. Remember, no special characters like hyphens or spaces are allowed in variable names.