Create Multibyte Safe Random String (PHP 7+)

Str.php

<?php

class Str
{

	/**
	 * Generate and return a random string having a specified length and composed of a specified
	 * set of characters.
	 * 
	 * <i>This method is multibyte safe.</i>
	 * 
	 * @param int $length [optional = 32]
	 * <div>The length of the random string.</div>
	 * 
	 * @param string $pool [optional]
	 * <div>List of characters used to generate the random string.</div>
	 * 
	 * @param string $encoding [optional]
	 * <div>The encoding of $pool.</div>
	 * 
	 * @return string
	 * @throws Str_Exception
	 */
	public static function random( int $length = 32, string $pool = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz', string $encoding = null ) : string
	{

		if( $length === 0 )
			return '';

		if( $length < 0 )
			throw new Str_Exception( 'Length must be >= 0.' );

		if( $pool === '' )
			throw new Str_Exception( 'Pool cannot be an empty string.' );

		$encoding = $encoding ?? mb_internal_encoding();
		$maxKey   = mb_strlen( $pool, $encoding );

		if( $maxKey === false )
			throw new Str_Exception( 'Invalid encoding specified.' );

		for( --$maxKey, $random = ''; $length-- ; )
			$random .= mb_substr( $pool, random_int( 0, $maxKey ), 1, $encoding );

		return $random;
	}

}

Str_Exception.php

<?php

class Str_Exception extends Exception{}

tests.php

<?php

$string   = 'a';
$mbString = 'ä';

assert( Str::random( 0     ) === '' );
assert( Str::random( 0, '' ) === '' );

assert( strlen( Str::random( 10 ) ) === 10 );

assert( Str::random( 1, $string ) === $string );
assert( Str::random( 2, $string ) === str_repeat( $string, 2 ) );

assert( Str::random( 1, $mbString ) === $mbString );
assert( Str::random( 2, $mbString ) === str_repeat( $mbString, 2 ) );

try
{
	Str::random( -1 );

	assert( false, 'Expecting Str_Exception for invalid length.' );
}
catch( Str_Exception $e ){}

try
{
	Str::random( 1, '' );

	assert( false, 'Expecting Str_Exception for empty pool.' );
}
catch( Str_Exception $e ){}

try
{
	@Str::random( 1, $mbString, '' );

	assert( false, 'Expecting Str_Exception for invalid encoding.' );
}
catch( Str_Exception $e ){}

This is a forked version of the fantastic random_str() sample provided by Scott Arciszewski. In particular, it:

1. Throws for invalid $lengths.
2. Throws for empty $pools.
3. Supports multibyte characters in $pool.