🧵 Strings — PHP
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PHP strings are immutable sequences of bytes. Most string operations return new strings. Strings can be treated like arrays for character access ($s[0]), but modification creates a new value.
Subtopics
| # | Topic | Description |
|---|
| 01 | String Basics | Creation, access, slicing, ASCII, common ops |
| 02 | PHP String Functions | strpos, str_replace, explode, preg_match and more |
| 03 | Patterns | Two pointers, sliding window, palindromes, KMP |
| 04 | Top 25 Interview Questions | Solved problems with approach sections |
The Big Picture
STRINGS IN PHP
│
├── Basics ← length, access, slice, ASCII
│ ├── strlen($s)
│ ├── $s[0], $s[$i]
│ ├── substr($s, start, len)
│ └── ord($c), chr($n)
│
├── Built-in Functions ← PHP standard library
│ ├── strpos / strrpos
│ ├── str_replace / str_repeat
│ ├── explode / implode
│ ├── strtolower / strtoupper / ucfirst
│ ├── trim / ltrim / rtrim
│ ├── str_split / str_contains
│ └── preg_match / preg_replace
│
├── Patterns ← DSA on strings
│ ├── Two Pointers ← reverse, palindrome, two-sum
│ ├── Sliding Window ← longest no-repeat, min window
│ ├── Hashing ← anagram, frequency count
│ └── Recursion ← palindrome partitioning
│
└── Interview Questions ← Top 25 problems
Complexity Cheat Sheet
| Operation | Time | Notes |
|---|
Length strlen($s) | O(n) | PHP recalculates each time |
Access $s[$i] | O(1) | Array-like access |
Concatenation . | O(n) | Creates new string |
Search strpos | O(n×m) | Naive search |
Reverse strrev | O(n) | Built-in |
Substring substr | O(k) | k = length of result |
Split explode / str_split | O(n) | |
Replace str_replace | O(n×m) | |
PHP vs JS String Syntax Comparison
| Operation | PHP | JavaScript |
|---|
| Length | strlen($s) | s.length |
| Access char | $s[$i] | s[i] |
| Substring | substr($s, 1, 3) | s.slice(1, 4) |
| Find | strpos($s, 'x') | s.indexOf('x') |
| Reverse | strrev($s) | s.split('').reverse().join('') |
| Split | str_split($s) | s.split('') |
| Join | implode('', $arr) | arr.join('') |
| Upper | strtoupper($s) | s.toUpperCase() |
| ASCII | ord($c) | s.charCodeAt(i) |
| Char from int | chr($n) | String.fromCharCode(n) |