strpos() / strrpos() / mb_strpos() Since: PHP 4（2000）

// Searches from the beginning and returns the position of the first match.
strpos($haystack, $needle, $offset);

// Searches from the end and returns the position of the last match.
strrpos($haystack, $needle, $offset);

// Searches from the beginning with multibyte character support.
mb_strpos($haystack, $needle, $offset, $encoding);

// Returns the portion of the string from the first match onward.
strstr($haystack, $needle, $before_needle);

// Searches case-insensitively.
stristr($haystack, $needle, $before_needle);

strpos(), strrpos(), and mb_strpos() return the position as an integer, or false if the substring is not found. strstr() and stristr() return the matched portion of the string, or false if not found.

<?php
$str = "東京都渋谷区神南1丁目";

// Search with strpos(). Returns a byte offset for multibyte strings.
echo strpos($str, "渋谷"); // Outputs the UTF-8 byte offset '9'.

// Use mb_strpos() to get the correct character position.
echo mb_strpos($str, "渋谷", 0, "UTF-8"); // Outputs '3'.

// Returns false if the substring is not found.
var_dump(mb_strpos($str, "大阪", 0, "UTF-8")); // Outputs 'bool(false)'.

// Use strrpos() to find the position of the last match.
$path = "/var/www/html/index.php";
echo strrpos($path, "/"); // Outputs '13'.

// Use strstr() to get the portion of the string from the match onward.
$email = "user@example.com";
echo strstr($email, "@"); // Outputs '@example.com'.

// Pass true as the third argument to get the portion before the match.
echo strstr($email, "@", true); // Outputs 'user'.

// stristr() is case-insensitive.
echo stristr("Hello World", "hello"); // Outputs 'Hello World'.

// Use strict comparison when checking the return value of strpos().
if (strpos("abcdef", "abc") !== false) {
	echo "Found."; // Works correctly even when the match is at position 0.
}

strpos() searches a string for a substring and returns the position of the first match. It returns false when the substring is not found, but returns 0 when found at the very beginning — so using == for comparison will not work correctly. Always use !== false for strict comparison.

For strings containing multibyte characters, strpos() returns a byte offset rather than a character position. Use mb_strpos() to get the correct character position.

If you simply need to check whether a string contains a substring, you can use str_contains() in PHP 8.0 or later. To extract a portion of a string starting from a specific character — such as getting the domain from an email address — strstr() is a convenient choice. To replace substrings, use str_replace().