What is the difference between single quotes and double quotes in PHP?

In PHP, both single and double quotes can be used to define strings. The main difference between them is as follows:

1. Variables and escape characters in single quoted strings are not parsed, whereas in double quoted strings they are parsed. This means that in double quoted strings, variable values can be used directly, while in single quoted strings, variables are treated as string literals.

For example:

$name = 'John';
echo 'My name is $name';  // 输出：My name is $name
echo "My name is $name";  // 输出：My name is John

1. Special characters in single quoted strings (except for the single quote itself) are interpreted as literals, whereas special characters in double quoted strings are interpreted as having a specific meaning.

For example:

echo 'Hello\nWorld';  // 输出：Hello\nWorld
echo "Hello\nWorld";  // 输出：Hello（换行）World

1. In double quoted strings, curly braces can be used to clearly define the boundaries of variables, while single quoted strings do not support this.

For instance,

$age = 25;
echo "I am {$age} years old.";  // 输出：I am 25 years old.

In conclusion, single quotation marks are mainly used for literals and simple string concatenation, while double quotation marks are more versatile and suitable for strings containing variables and special characters.