Substring extraction in PHP - Step-by-Step Execution

Concept Flow - Substring extraction

Start with full string

↓

Specify start position

↓

Specify length (optional)

↓

Extract substring

↓

Return substring

The program starts with a full string, then chooses where to start and how many characters to take, finally extracting and returning that substring.

Execution Sample

PHP

<?php
$str = "Hello, world!";
$sub = substr($str, 7, 5);
echo $sub;
?>

Extracts 5 characters from position 7 of the string "Hello, world!" and prints it.

Execution Table

Step	Action	Input String	Start Position	Length	Extracted Substring	Output
1	Define string	Hello, world!	-	-	-	-
2	Call substr	Hello, world!	7	5	-	-
3	Extract substring	Hello, world!	7	5	world	-
4	Print substring	Hello, world!	7	5	world	world
5	End	-	-	-	-	-

💡 Extraction done and substring printed; program ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
$str	"Hello, world!"	"Hello, world!"	"Hello, world!"	"Hello, world!"	"Hello, world!"
$sub	undefined	undefined	"world"	"world"	"world"

Key Moments - 3 Insights

Why does the substring start at position 7, not 6?

In PHP, string positions start at 0, so position 7 means the 8th character. The execution_table row 2 shows start position 7 used in substr.

What happens if length is omitted?

If length is omitted, substr extracts from start position to the end of the string. This is not shown here but is a common behavior.

Why is the output 'world' and not 'world!'?

Length 5 means exactly 5 characters are taken starting at position 7. The exclamation mark is at position 12, outside the 5 characters. See execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $sub after step 3?

A "world!"

B "Hello"

C "world"

D "orld!"

💡 Hint

Check the 'Extracted Substring' column in row 3 of execution_table.

At which step is the substring printed?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Action' and 'Output' columns in execution_table.

If the start position was changed to 0 and length to 5, what would $sub be?

A "Hello"

B "world"

C "Hello,"

D "ello,"

💡 Hint

Think about extracting 5 characters from the start of the string "$str".

Concept Snapshot

substr(string, start, length) extracts a substring from 'string'.
Start counts from 0.
Length is optional; if omitted, substring goes to end.
Returns the extracted substring.
Used to get parts of strings easily.

Full Transcript

This example shows how PHP's substr function extracts a part of a string. We start with the string "Hello, world!". We choose position 7 as the start, which is the 8th character because counting starts at zero. We take 5 characters from there. The substring extracted is "world". Then we print it. The program ends after printing. This helps us get any part of a string by specifying where to start and how many characters to take.