PHPの一重引用符と二重引用符の使用法

$s = "I am a 'single quote string' inside a double quote string"; 
$s = 'I am a "double quote string" inside a single quote string'; 
$s = "I am a 'single quote string' inside a double quote string"; 
$s = 'I am a "double quote string" inside a single quote string'; 　　

$full_name = $first_name . ' ' . $last_name; 
$full_name = "$first_name $last_name"; 　

$foo = 2; 
echo "foo is $foo"; // 打印结果: foo is 2 
echo 'foo is $foo'; // 打印结果: foo is $foo 
echo "foo is $foo\n"; // 打印结果: foo is 2 (同时换行) 
echo 'foo is $foo\n'; // 打印结果: foo is $foo\n 
$foo = 2; 
echo "foo is $foo"; // 打印结果: foo is 2 
echo 'foo is $foo'; // 打印结果: foo is $foo 
echo "foo is $foo\n"; // 打印结果: foo is 2 (同时换行) 
echo 'foo is $foo\n'; // 打印结果: foo is $foo\n 　　

echo "value = $foo"; 
echo "value = $a[$i]"; 
echo "value = $foo"; 
echo "value = $a[$i]"; 　　

echo "value = $a[$i][$j]"; //我们希望打印二维数组$a的某个元素。 　　

echo 'value = ' . $a[$i][$j];//字符串的连接用点(.)

echo "value = {$a[$i][$j]}" //打印二维数组$a的某个元素 　　

$sql="insert into userinfo (username,password) Values('O'Kefee','123456')" 　

$s = addslashes("O'Kefee") ……Values('".$s."',…… 　　

$abc='I love u'; 
echo $abc //结果是:I love u 
echo '$abc' //结果是:$abc 
echo "$abc" //结果是:I love u

select * from table where user='abc';

$user='abc'; 
SQL1="select * from table where user=' ".$user." ' ";对比一下 
SQL2="select * from table where user=' abc ' "