Strings

Double-quoted strings

Support escape sequences:

<?php
echo "Hello\n";      // newline
echo "Tab\there";    // tab
echo "Return\r";     // carriage return
echo "Vert\v";       // vertical tab
echo "Esc\e";        // escape byte
echo "Form\f";       // form feed
echo "Quote: \"";    // escaped quote
echo "Backslash: \\"; // backslash
echo "\x41";         // hex byte: A
echo "\101";         // octal byte: A
echo "\u{1F600}";    // Unicode codepoint: 😀

Single-quoted strings

No escape sequences except \\ and \':

<?php
echo 'Hello\n';      // prints: Hello\n (literal)
echo 'It\'s here';   // prints: It's here

String interpolation

<?php
$name = "World";
echo "Hello, $name\n";

Heredoc strings

Multi-line with escape processing (like double-quoted):

<?php
echo <<<EOT
Hello World
This is line 2
EOT;

Nowdoc strings

Multi-line without escape processing (like single-quoted):

<?php
echo <<<'EOT'
Hello World
No escapes: \n \t stay literal
EOT;

String indexing

<?php
$s = "hello";
echo $s[1];    // e
echo $s[-1];   // o
echo "[" . $s[99] . "]";  // []

Read-only. Negative indices count from end. Out-of-bounds returns empty string.

Built-in string functions

Function	Signature	Description
strlen()	strlen($str): int	Returns string length
substr()	substr($str, $start [, $len]): string	Extract substring
strpos()	strpos($hay, $needle): int|false	Find first occurrence. Returns false if not found
strrpos()	strrpos($hay, $needle): int|false	Find last occurrence. Returns false if not found
strstr()	strstr($hay, $needle): string	Find first occurrence and return rest
str_replace()	str_replace($search, $replace, $subject): string	Replace all occurrences
str_ireplace()	str_ireplace($search, $replace, $subject): string	Case-insensitive replace
substr_replace()	substr_replace($str, $repl, $start [, $len]): string	Replace substring
strtolower()	strtolower($str): string	Convert to lowercase
strtoupper()	strtoupper($str): string	Convert to uppercase
ucfirst()	ucfirst($str): string	Uppercase first character
lcfirst()	lcfirst($str): string	Lowercase first character
ucwords()	ucwords($str): string	Uppercase first letter of each word
trim()	trim($str [, $chars]): string	Strip whitespace from both ends
ltrim()	ltrim($str [, $chars]): string	Strip whitespace from left
rtrim()	rtrim($str [, $chars]): string	Strip whitespace from right
str_repeat()	str_repeat($str, $times): string	Repeat a string
str_pad()	str_pad($str, $len [, $pad, $type]): string	Pad string to length
str_split()	str_split($str [, $len]): array	Split into chunks
strrev()	strrev($str): string	Reverse a string
strcmp()	strcmp($a, $b): int	Binary-safe string comparison
strcasecmp()	strcasecmp($a, $b): int	Case-insensitive comparison
str_contains()	str_contains($hay, $needle): bool	Check if string contains substring
str_starts_with()	str_starts_with($hay, $prefix): bool	Check prefix
str_ends_with()	str_ends_with($hay, $suffix): bool	Check suffix
ord()	ord($char): int	ASCII value of first character
chr()	chr($code): string	Character from ASCII code
explode()	explode($delim, $str): array	Split string into array
implode()	implode($glue, $arr): string	Join array into string
number_format()	number_format($n [, $dec [, $dec_point, $thou_sep]]): string	Format number
sprintf()	sprintf($fmt, ...): string	Format string (%s, %d, %f, %x, %e, %g, %o, %c, %%)
printf()	printf($fmt, ...): int	Format and print
sscanf()	sscanf($str, $fmt): array	Parse string with format (%d, %s)
addslashes()	addslashes($str): string	Escape quotes and backslashes
stripslashes()	stripslashes($str): string	Remove escape backslashes
nl2br()	nl2br($str): string	Insert <br /> before newlines
wordwrap()	wordwrap($str [, $width [, $break [, $cut]]]): string	Wrap text at width
bin2hex()	bin2hex($str): string	Convert binary to hex
hex2bin()	hex2bin($str): string	Convert hex to binary
md5()	md5($str): string	MD5 hash (32-char hex)
sha1()	sha1($str): string	SHA1 hash (40-char hex)
hash()	hash($algo, $data): string	Hash with algorithm (md5, sha1, sha256)
htmlspecialchars()	htmlspecialchars($str): string	Escape HTML special chars
htmlentities()	htmlentities($str): string	Alias for htmlspecialchars
html_entity_decode()	html_entity_decode($str): string	Decode HTML entities
urlencode()	urlencode($str): string	URL-encode (spaces as +)
urldecode()	urldecode($str): string	URL-decode
rawurlencode()	rawurlencode($str): string	URL-encode (spaces as %20)
rawurldecode()	rawurldecode($str): string	URL-decode (RFC 3986)
base64_encode()	base64_encode($str): string	Base64 encode
base64_decode()	base64_decode($str): string	Base64 decode
ctype_alpha()	ctype_alpha($str): bool	All chars are A-Z/a-z
ctype_digit()	ctype_digit($str): bool	All chars are 0-9
ctype_alnum()	ctype_alnum($str): bool	All chars are alphanumeric
ctype_space()	ctype_space($str): bool	All chars are whitespace
preg_match()	preg_match($pattern, $subject): int	Test if regex matches (1 or 0). Uses POSIX extended regex.
preg_match_all()	preg_match_all($pattern, $subject): int	Count all non-overlapping matches
preg_replace()	preg_replace($pattern, $replacement, $subject): string	Replace all regex matches; supports $0..$9 and \0..\9 replacement backreferences
preg_replace_callback()	preg_replace_callback($pattern, $callback, $subject): string	Replace all regex matches with the callback return value; callback receives array<string> matches
preg_split()	preg_split($pattern, $subject): array	Split string by regex pattern

Regex limitations

• Uses POSIX extended regex via libc, with translation of common PCRE shorthands (\s, \d, \w) and common Unicode property shims (\p{L}, \p{N}, \p{Lu}, \p{Ll}, and \P{...})
• preg_replace() expands $0..$9 and \0..\9 to captured groups; unmatched optional groups expand to an empty string
• preg_replace_callback() passes $matches[0] as the full match and $matches[1]..$matches[9] as supported capture groups
• Lookahead, lookbehind, non-greedy quantifiers are not supported
• preg_match() does not support $matches capture parameter