Unexpected result of greater than or less than comparison on PHP 8

The below returns false on PHP 7 but true on PHP 8. Could someone explain why this is happening?

var_dump("U0M262" > 100000);

Solution

There is no obviously correct result for a comparison between a string and a number. In many languages, it would just give an error; in others, including PHP, the language tries to make sense of it by converting both operands to the same type, but this involves a judgement of which type to "prefer".

Historically, PHP has preferred comparing numbers to comparing strings: it treated "U0M262" > 100000 as (int)"U0M262" > 100000. Since (int)"U0M262" has no obvious value, it is evaluated as 0, and the expression becomes 0 > 100000, which is false.

As of PHP 8, this behaviour has changed and PHP now only uses a numeric comparison for "numeric strings", e.g. "42" clearly "looks like" 42.

Since "U0M262" doesn't fit the requirements for a numeric string, "U0M262" > 100000 is now treated as "U0M262" > (string)100000. This does a byte-wise comparison of the sort order for the two strings, and finds that since "U" comes after "1" in ASCII (and any ASCII-derived encoding, including UTF-8), the result is true.

Because of how ASCII (and compatible encodings such as UTF-8) is arranged:

A string starting with a control character or space will be "less than" any number
A string starting with a letter will be "more than" any number
A string starting with any of "! " # $ % & ' ( ) * + , - . /" will be "less than" any number
For a string starting with a digit, you need to look at the individual bytes
Any other string will be "more than" any number

As ever, you can tell PHP which comparison you intended, and get the correct behaviour in all versions, using explicit casts:

var_dump((int)"U0M262" > (int)100000); // bool(false)
var_dump((string)"U0M262" > (string)100000); // bool(true)

(Obviously, this makes no sense if you're hard-coding both sides anyway, but assuming one or both is a variable, this is how you'd do it.)