ParagonIE_Sodium_Core_Util::substr( string $str, int $start = 0, int $length = null ): string

Compatibility

Present in every tracked release (6.7.7 to 7.1.0), and compiles on PHP 7.4 through 8.6-dev.

Parameters

$strstring

$startintoptional

Default: 0

$lengthintoptional

Default: null

Return value

string

Code examples

Every example is editable and runs in a real WordPress booted in your browser by WordPress Playground. Press Run, then edit the code: clicking away re-runs it. Nothing is sent anywhere until you do.

$binary = random_bytes( 32 );

$first_half  = ParagonIE_Sodium_Core_Util::substr( $binary, 0, 16 );
$second_half = ParagonIE_Sodium_Core_Util::substr( $binary, 16 );

printf(
	'First half: %d bytes, second half: %d bytes',
	strlen( $first_half ),
	strlen( $second_half )
);

try {
	$result = ParagonIE_Sodium_Core_Util::substr( 12345, 0, 3 );
	echo esc_html( $result );
} catch ( TypeError $e ) {
	echo esc_html( 'Caught TypeError: ' . $e->getMessage() );
}

Common problems and fixes · 3

• Why do I get a TypeError instead of a warning when I pass the wrong type?
• Why does passing $length as 0 give me an empty string instead of the rest of the string?
• Will this behave differently on servers with mbstring.func_overload enabled?

Alternatives and related functions

substr

When you are slicing a normal ASCII or already byte-safe string outside of a cryptographic context and don't need mbstring.func_overload protection.

mb_substr

When you are deliberately working with multibyte text (like UTF-8 user content) and want character-aware slicing rather than byte-aware slicing.

ParagonIE_Sodium_Core_Util::strlen

When you need the byte length of a string before or after calling substr(), for example to compute a $length argument safely.

Performance profile

How much work a call to ParagonIE_Sodium_Core_Util::substr() does, and what it touches: the algorithmic scaling, the Zend instruction count per call across PHP versions, the hooks it hands control to, and the core code that calls it. Measured from the compiled opcodes, not a stopwatch, so every number is identical on any machine running the same PHP version, and every function in core is ranked by cost.

Cost class

Trivial

Touches nothing outside its own arguments.

Scaling

Constant

No loop in the body: the same number of instructions runs whatever you pass in.

Instructions

8–21

Executed per call on PHP 8.5, depending on the branch taken. The body compiles to 35.

Plugin surface

None

Nothing here hands control to plugin code.

Called by

46

46 places in core call this, so the cost is paid more often than your own code shows.

What one call costs · 3 distinct outcomes

One number would be a lie: the work depends on which branch runs. These are every distinct cost ParagonIE_Sodium_Core_Util::substr() can have, taken from its control-flow graph on PHP 8.5.

Uses · 4

• mb_substr()Compat function to mimic mb_substr().
• TypeError::__construct()
• ParagonIE_Sodium_Core_Util::isMbStringOverride()Returns whether or not mbstring.func_overload is in effect.
• ParagonIE_Sodium_Core_Util::strlen()Safe string length

Used by · 46

• ParagonIE_Sodium_Compat::crypto_aead_aegis128l_decrypt()Authenticated Encryption with Associated Data: Decryption
• ParagonIE_Sodium_Compat::crypto_aead_aegis256_decrypt()Authenticated Encryption with Associated Data: Decryption
• ParagonIE_Sodium_Compat::crypto_aead_aes256gcm_decrypt()Authenticated Encryption with Associated Data: Decryption
• ParagonIE_Sodium_Compat::crypto_kx_client_session_keys()
• ParagonIE_Sodium_Compat::crypto_kx_publickey()
• ParagonIE_Sodium_Compat::crypto_kx_secretkey()
• ParagonIE_Sodium_Compat::crypto_kx_server_session_keys()
• ParagonIE_Sodium_Compat::crypto_sign_ed25519_sk_to_curve25519()Convert an Ed25519 secret key to a Curve25519 secret key
• ParagonIE_Sodium_Compat::increment()Increase a string (little endian)
• ParagonIE_Sodium_Compat::unpad()
• ParagonIE_Sodium_Core_AEGIS_State128L::absorb()
• ParagonIE_Sodium_Core_AEGIS_State128L::dec()

Source code

View on Trac ↗View on GitHub ↗

836    public static function substr($str, $start = 0, $length = null)837    {838        /* Type checks: */839        if (!is_string($str)) {840            throw new TypeError('String expected');841        }842 843        if ($length === 0) {844            return '';845        }846 847        if (self::isMbStringOverride()) {848            if (PHP_VERSION_ID < 50400 && $length === null) {849                $length = self::strlen($str);850            }851            $sub = (string) mb_substr($str, $start, $length, '8bit');852        } elseif ($length === null) {853            $sub = (string) substr($str, $start);854        } else {855            $sub = (string) substr($str, $start, $length);856        }857        if ($sub !== '') {858            return $sub;859        }860        return '';861    }

Changelog

Unchanged from 6.7.7 through 7.1.0.

1. 6.7.7
2. 6.8.8
3. 6.9.7
4. 7.0.4
5. 7.1.0

Signature, return type and hooks compared across 5 parsed releases.

About this page

Parsed data

Generated from the wordpress-develop 7.1.0 tag, from src/wp-includes/sodium_compat/src/Core/Util.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.

Corrections

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