Fall back on an HTML mode for Blade when web-mode is missing

[zonuexe]

Answers "can we fall back on html-mode when web-mode isn't installed?" — yes, and it also fixes a hard failure.

The bug

Opening a .blade.php without web-mode installed didn't warn and carry on. It failed:

Warning: php-mode is NOT support blade template. Please install `web-mode' package
Debugger entered--Lisp error: (void-function web-mode)

php-derivation-major-mode warned but left MODE set to the unavailable web-mode, so php-mode-maybe funcalled it. Every other template zeroes MODE and falls back on php-default-major-mode — Blade was the one case that couldn't open at all.

The fix

Degrade instead. A Blade template is mostly HTML plus Blade's own directives — it isn't PHP, so php-mode can't read it either — and an HTML mode renders the bulk of it correctly.

New php-blade-template-major-mode-fallback, default '(mhtml-mode html-mode) (both built in, no new dependency). The first available entry wins; set it to nil to opt out and get php-default-major-mode as before. The warning now names the substitute rather than claiming Blade is unsupported:

Warning: ‘web-mode’ is not available for this Blade template; using ‘mhtml-mode’ instead.
Install the ‘web-mode’ package for full Blade support.

Scope: Blade only, deliberately

The fallback does not extend to php-html-template-major-mode. That mode is shared with the auto-detection path, and php-project-php-file-as-template defaults to auto — so any .php file containing an HTML tag reaches it. Routing those to an HTML mode would take most PHP files away from php-mode for everyone without web-mode. Those templates are PHP with HTML in them, which php-mode does read, so they keep falling back on php-default-major-mode.

php-mode-test-html-template-fallback-unchanged pins that down — it passes both before and after this change.

Verification

End-to-end, without web-mode installed:

file	before	after
welcome.blade.php	void-function web-mode	mhtml-mode
page.php (has <div>)	php-mode	php-mode (unchanged)
plain.php	php-mode	php-mode (unchanged)

The Blade test fails before the fix and passes after; the HTML-template test passes both ways. Byte-compiles with no new warnings; full suite green. seq-find is deliberately avoided since seq isn't preloaded on every supported Emacs, so this is safe for the Emacs 27-supporting series.

For context: how Emacs's built-in php-ts-mode handles Blade

It doesn't — there is no mention of Blade anywhere in the Emacs tree. .blade.php ends in .php, so it matches php-ts-mode's auto-mode-alist patterns and opens as plain php-ts-mode, where @extends and {{ }} are parse errors. This package routes Blade to web-mode and now degrades sensibly without it, so it stays ahead of core on that front either way.