How does PHP treat different values in boolean context?

I wonder how PHP handles true/false comparison internally. I understand that true is defined as 1 and false is defined as 0. When I do if("a"){ echo "true";} it echos "true". How does PHP recognize "a" as 1 ?

This is covered in the PHP documentation for booleans and type comparison tables.

When converting to boolean, the following values are considered FALSE:

• the boolean FALSE itself
• the integer 0 (zero)
• the float 0.0 (zero)
• the empty string, and the string '0'
• an array with zero elements
• an object with zero member variables (PHP 4 only)
• the special type NULL (including unset variables)
• SimpleXML objects created from empty tags

Every other value is considered TRUE.