$query = 'SELECT '.$this->select_fields.' FROM '.$this->tablename.
         ' WHERE '.$this->pkey_field.'=\''.$this->pkey_value.'\'';

$query = "SELECT $this->select_fields FROM $this->tablename WHERE $this->pkey_field='$this->pkey_value'";

<?php

// test of parsing time

require 'myclasses/timer.class.php';

class DynamicSql {

	var $select_fields;
	var $pkey_field;
	var $pkey_value;
	var $tablename;
	
	// constructor
	
	function DynamicSql($tablename, $pkey_field, $pkey_value, $select_fields)
	{
		$this->tablename 	 = $tablename;
		$this->pkey_field 	 = $pkey_field;
		$this->pkey_value	 = $pkey_value;
		$this->select_fields = $select_fields;
	}
	
	// returns select sql query (variant  1 )
	
	function getDynamicSqlQuery1()
	{
		return 'SELECT '.$this->select_fields.' FROM '.$this->tablename.' WHERE '.$this->pkey_field.'=\''.$this->pkey_value.'\'';
	}
	
	// returns select sql query (variant  2 )
		
	function getDynamicSqlQuery2()
	{
		return "SELECT $this->select_fields FROM $this->tablename WHERE $this->pkey_field='$this->pkey_value'";
	}
}

$sql = new DynamicSql('tmp_table', 'id',  1 , 'id, foo, bar');

$t1 = new CTimer();

echo $sql->getDynamicSqlQuery1().'<br>';

$t1->stop();
printf('<u>Parsing time for getDynamicSqlQuery1()</u>: <b>%f</b> sec.', $t1->getPassedTime());

echo '<br><br>';

$t2 = new CTimer();

echo $sql->getDynamicSqlQuery2().'<br>';

$t2->stop();
printf('<u>Parsing time for getDynamicSqlQuery2()</u>: <b>%f</b> sec.', $t2->getPassedTime());

?>

SELECT id, foo, bar FROM tmp_table WHERE id='1'
Parsing time for getDynamicSqlQuery1():  0 . 000187  sec.

SELECT id, foo, bar FROM tmp_table WHERE id='1'
Parsing time for getDynamicSqlQuery2():  0 . 000041  sec.