当然ですが、 eval() は使いません。

class calculation {
	public function __construct() {
	}
	
	// 乗算 除算
	private function multiplication_division( $interval ) {
		while ( preg_match ( '@([-0-9]+)\s*(\*|/)\s*([-0-9]+)@', $interval, $r ) ) {
			if ( $r[2] == '*' )
				$interval = str_replace ( $r[0], (int) $r[1] * (int) $r[3], $interval );
			elseif ( $r[2] == '/' )
				$interval = str_replace ( $r[0], (int) $r[1] / (int) $r[3], $interval );
		}
		return $interval;
	}
	
	// 加算 減算
	private function addition_subtraction( $interval ) {
		while ( preg_match ( '@([-0-9]+)\s*(\+|-)\s*([-0-9]+)@', $interval, $r ) ) {
			if ( $r[2] == '+' )
				$interval = str_replace ( $r[0], (int) $r[1] + (int) $r[3], $interval );
			elseif ( $r[2] == '-' )
				$interval = str_replace ( $r[0], (int) $r[1] - (int) $r[3], $interval );
		}
		return $interval;
	}
	
	public function calculation( $interval ) {
		while ( preg_match ( '@\(([^\(\)]+)\)@', $interval, $r ) ) {
			$formula = $r[1];
			$formula = $this->multiplication_division( $formula );
			$formula = $this->addition_subtraction( $formula );
			$interval = str_replace ( $r[0], $formula, $interval );
		}
		$interval = $this->multiplication_division( $interval );
		$interval = $this->addition_subtraction( $interval );
		return $interval;
	}
}

$interval = '( ( 2 + 3 - ( 2 * 2 + 6 - 5 ) ) + 10 * 5 / ( 4 - 2 ) ) * 3';
$calc = new calculation;
echo $calc->calculation( $interval );

まずは () の計算を加算＆減算→乗算＆除算の順番で行います。
() をその計算結果に置き換えています。

while ( preg_match ( '@\(([^\(\)]+)\)@', $interval, $r ) ) {
	$formula = $r[1];
	$formula = $this->multiplication_division( $formula );
	$formula = $this->addition_subtraction( $formula );
	$interval = str_replace ( $r[0], $formula, $interval );
}

あとは加算＆減算→乗算＆除算でおしまいです。

$interval = $this->multiplication_division( $interval );
$interval = $this->addition_subtraction( $interval );